[PATCH] diffcore-pickaxe doc: document -S and -G properly

[Ramkumar Ramachandra <artagnon@gmail.com>]

The documentation of -S and -G is very sketchy.  Completely rewrite the
sections in Documentation/diff-options.txt and
Documentation/gitdiffcore.txt.

References:
52e9578 ([PATCH] Introducing software archaeologist's tool "pickaxe".)
f506b8e (git log/diff: add -G<regexp> that greps in the patch text)

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 I spent some time reading the code and history to figure out what -S
 and -G really do.  I hope I've done justice.

 Documentation/diff-options.txt | 35 +++++++++++++++++++++++++++-------
 Documentation/gitdiffcore.txt  | 43 +++++++++++++++++++++++-------------------
 2 files changed, 52 insertions(+), 26 deletions(-)

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 104579d..765abc5 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -383,14 +383,35 @@ ifndef::git-format-patch[]
 	that matches other criteria, nothing is selected.
 
 -S<string>::
-	Look for differences that introduce or remove an instance of
-	<string>. Note that this is different than the string simply
-	appearing in diff output; see the 'pickaxe' entry in
-	linkgit:gitdiffcore[7] for more details.
+	Look for commits where the specified string was added or
+	removed.  More precisely, find commits that change the number
+	of occurrences of the specified string.
++
+It is often useful when you're looking for an exact string (like a
+function prototype), and want to know the history of that string since
+it first came into being.
 
 -G<regex>::
-	Look for differences whose added or removed line matches
-	the given <regex>.
+	Grep through the patch text of commits for added/removed lines
+	that match <regex>.  `--pickaxe-regex` is implied in this
+	mode.
++
+To illustrate the difference between `-S<regex> --pickaxe-regex` and
+`-G<regex>`, consider a commit with the following diff in the same
+file:
++
+----
++    return !regexec(regexp, two->ptr, 1, &regmatch, 0);
+...
+-    hit = !regexec(regexp, mf2.ptr, 1, &regmatch, 0);
+----
++
+While `git log -G"regexec\(regexp"` will show this commit, `git log
+-S"regexec\(regexp" --pickaxe-regex` will not (because the number of
+occurrences of that string didn't change).
++
+See the 'pickaxe' entry in linkgit:gitdiffcore[7] for more
+information.
 
 --pickaxe-all::
 	When `-S` or `-G` finds a change, show all the changes in that
@@ -399,7 +420,7 @@ ifndef::git-format-patch[]
 
 --pickaxe-regex::
 	Make the <string> not a plain string but an extended POSIX
-	regex to match.
+	regex to match.  Implied when using `-G`.
 endif::git-format-patch[]
 
 -O<orderfile>::
diff --git a/Documentation/gitdiffcore.txt b/Documentation/gitdiffcore.txt
index 568d757..39b9c51 100644
--- a/Documentation/gitdiffcore.txt
+++ b/Documentation/gitdiffcore.txt
@@ -222,25 +222,30 @@ version prefixed with '+'.
 diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
 ---------------------------------------------------------------------
 
-This transformation is used to find filepairs that represent
-changes that touch a specified string, and is controlled by the
--S option and the `--pickaxe-all` option to the 'git diff-*'
-commands.
-
-When diffcore-pickaxe is in use, it checks if there are
-filepairs whose "result" side and whose "origin" side have
-different number of specified string.  Such a filepair represents
-"the string appeared in this changeset".  It also checks for the
-opposite case that loses the specified string.
-
-When `--pickaxe-all` is not in effect, diffcore-pickaxe leaves
-only such filepairs that touch the specified string in its
-output.  When `--pickaxe-all` is used, diffcore-pickaxe leaves all
-filepairs intact if there is such a filepair, or makes the
-output empty otherwise.  The latter behaviour is designed to
-make reviewing of the changes in the context of the whole
-changeset easier.
-
+There are two kinds of pickaxe: the S kind (corresponding to 'git log
+-S') and the G kind (corresponding to 'git log -G').
+
+The S kind detects filepairs whose "result" side and "origin" side
+have different number of occurrences of specified string.  While
+rename detection works as usual, 'git log -S' cannot omit commits
+where a the small string being looked for is moved verbatim from one
+file to another (since the number of occurrences of that string
+changed in each of those two filepairs). The implementation
+essentially runs a count, and is significantly cheaper than the G
+kind.
+
+The G kind detects filepairs whose patch text has an added or a
+deleted line that matches the given regexp.  This means that it can
+detect in-file (or what rename-detection considers the same file)
+moves.  The implementation of 'git log -G' runs diff twice and greps,
+and this can be quite expensive.
+
+A diffcore-pickaxe option worth mentioning: `--pickaxe-all`.  When not
+in effect, diffcore-pickaxe leaves only such filepairs that touch the
+specified string in its output.  When in effect, diffcore-pickaxe
+leaves all filepairs intact if there is such a filepair, or makes the
+output empty otherwise.  The latter behavior is designed to make
+reviewing of the changes in the context of the whole changeset easier.
 
 diffcore-order: For Sorting the Output Based on Filenames
 ---------------------------------------------------------
-- 
1.8.3.rc1.61.g2cacfff