Documentation: point to related commands from gitignore

A frequently asked question on #git is how to stop tracking a file
that is mistakenly tracked by git.  A frequently attempted strategy is
to add such files to .gitignore.

Thus one might imagine that the gitignore documentation could be a
good entry point for 'git rm' documentation.  Add some
cross-references in this vein.

While at it, move a reference to update-index --assume-unchanged from
the DESCRIPTION to lower down on the page.  This way, the methodical
reader can benefit from first learning what excludes files do, then
how they relate to other git facilities.

Based-on-patch-by: Sitaram Chamarty <sitaram@atc.tcs.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitignore.txt

@@ -14,11 +14,8 @@ DESCRIPTION
 
 A `gitignore` file specifies intentionally untracked files that
 git should ignore.
-Note that all the `gitignore` files really concern only files
-that are not already tracked by git;
-in order to ignore uncommitted changes in already tracked files,
-please refer to the 'git update-index --assume-unchanged'
-documentation.
+Files already tracked by git are not affected; see the NOTES
+below for details.
 
 Each line in a `gitignore` file specifies a pattern.
 When deciding whether to ignore a path, git normally checks
@@ -99,6 +96,18 @@ PATTERN FORMAT
    For example, "/{asterisk}.c" matches "cat-file.c" but not
    "mozilla-sha1/sha1.c".
 
+NOTES
+-----
+
+The purpose of gitignore files is to ensure that certain files
+not tracked by git remain untracked.
+
+To ignore uncommitted changes in a file that is already tracked,
+use 'git update-index {litdd}assume-unchanged'.
+
+To stop tracking a file that is currently tracked, use
+'git rm --cached'.
+
 EXAMPLES
 --------
 
@@ -142,6 +151,11 @@ Another example:
 The second .gitignore prevents git from ignoring
 `arch/foo/kernel/vmlinux.lds.S`.
 
+SEE ALSO
+--------
+linkgit:git-rm[1], linkgit:git-update-index[1],
+linkgit:gitrepository-layout[5]
+
 Documentation
 -------------
 Documentation by David Greaves, Junio C Hamano, Josh Triplett,