doc: rephrase the purpose of the staging area

Git does not really "store the contents of the next commit"
anywhere; rather, you the user use the index to prepare it.

Signed-off-by: Julia Evans <julia@jvns.ca>
[jc; made the change relative to what is already in 'next']
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jvns

Documentation/git-add.adoc

@@ -17,8 +17,8 @@ git add [--verbose | -v] [--dry-run | -n] [--force | -f] [--interactive | -i] [-
 DESCRIPTION
 -----------
 Add contents of new or changed files to the index. The "index" (also
-known as "staging area") is where Git stores the contents of the next
-commit.
+known as the "staging area") is what you use to prepare the contents of
+the next commit.
 
 When you run `git commit` without any other arguments, it will only
 commit staged changes. For example, if you've edited `file.c` and want