[PATCH] Documentation/gitignore.txt: Clarify gitignore vs tracked files

[Petr Baudis <pasky@suse.cz>]

Explain more carefully that `.gitignore` concerns only untracked files
and there is no way to make git ignore local changes in already tracked
files.

This is currently probably one of the top FAQs at #git and the documentation
could be more explicit about this.

Signed-off-by: Petr Baudis <pasky@suse.cz>
---

 Documentation/gitignore.txt |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/Documentation/gitignore.txt b/Documentation/gitignore.txt
index fc0efd8..ac3c776 100644
--- a/Documentation/gitignore.txt
+++ b/Documentation/gitignore.txt
@@ -13,9 +13,12 @@ DESCRIPTION
 -----------
 
 A `gitignore` file specifies intentionally untracked files that
-git should ignore.  Each line in a `gitignore` file specifies a
-pattern.
+git should ignore.
+Note that all the `gitignore` files really concern only files
+that are not already tracked by git; there is no direct mechanism
+to make git ignore your local modifications in already tracked files.
 
+Each line in a `gitignore` file specifies a pattern.
 When deciding whether to ignore a path, git normally checks
 `gitignore` patterns from multiple sources, with the following
 order of precedence, from highest to lowest (within one level of