[PATCH v2] merge-options.txt: clarify meaning of various ff-related options

[Elijah Newren <newren@gmail.com>]

As discovered on the mailing list, some of the descriptions of the
ff-related options were unclear.  Try to be more precise with what these
options do.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
Changes since v1:
  * Grouped much like --option/--no-option items are to make it clearer
    that these are related options, as suggested by Sergey.

 Documentation/merge-options.txt | 33 ++++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/Documentation/merge-options.txt b/Documentation/merge-options.txt
index 79a00d2a4a..348d66f54b 100644
--- a/Documentation/merge-options.txt
+++ b/Documentation/merge-options.txt
@@ -39,21 +39,28 @@ set to `no` at the beginning of them.
 	to `MERGE_MSG` before being passed on to the commit machinery in the
 	case of a merge conflict.
 
---ff::
-	When the merge resolves as a fast-forward, only update the branch
-	pointer, without creating a merge commit.  This is the default
-	behavior.
-
+--ff-only::
 --no-ff::
-	Create a merge commit even when the merge resolves as a
-	fast-forward.  This is the default behaviour when merging an
-	annotated (and possibly signed) tag that is not stored in
-	its natural place in 'refs/tags/' hierarchy.
+--ff::
+	Whether to only allow resolving the merge as a fast forward
+	(only updating the branch pointer to match the merged branch
+	and not creating a merge commit), to never allow it (always
+	creating a merge commit), or to prefer it when possible.  The
+	default is --ff, except when merging an annotated (and
+	possibly signed) tag that is not stored in its natural place
+	in 'refs/tags/' hierarchy (in which case --no-ff is the
+	default).
++
+With --ff-only, resolve the merge as a fast-forward when possible
+(when the merged branch contains the current branch in its history).
+When not possible, refuse to merge and exit with a non-zero status.
++
+With --no-ff, create a merge commit in all cases, even when the merge
+could instead resolve as a fast-forward.
++
+With --ff, resolve the merge as a fast-forward when possible.  When not
+possible, create a merge commit.
 
---ff-only::
-	Refuse to merge and exit with a non-zero status unless the
-	current `HEAD` is already up to date or the merge can be
-	resolved as a fast-forward.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
-- 
2.23.0.9.g568fda2d03