replace reference to git-rm with git-reset in git-commit doc

The message in git-commit suggesting to use 'git rm --cached'
to unstage is just plain wrong. It really should mention 'git reset'.

Suggested by Jan Hudec.
Signed-off-by: NJing Xue <jingxue@digizenstudio.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -224,6 +224,7 @@ See Also
 --------
 gitlink:git-status[1]
 gitlink:git-rm[1]
+gitlink:git-reset[1]
 gitlink:git-mv[1]
 gitlink:git-commit[1]
 gitlink:git-update-index[1]

Documentation/git-commit.txt

@@ -154,10 +154,13 @@ EXAMPLES
 --------
 When recording your own work, the contents of modified files in
 your working tree are temporarily stored to a staging area
-called the "index" with gitlink:git-add[1].  Removal
-of a file is staged with gitlink:git-rm[1].  After building the
-state to be committed incrementally with these commands, `git
-commit` (without any pathname parameter) is used to record what
+called the "index" with gitlink:git-add[1].  A file can be
+reverted back, only in the index but not in the working tree,
+to that of the last commit with `git-reset HEAD -- <file>`,
+which effectively reverts `git-add` and prevents the changes to
+this file from participating in the next commit.  After building
+the state to be committed incrementally with these commands,
+`git commit` (without any pathname parameter) is used to record what
 has been staged so far.  This is the most basic form of the
 command.  An example: