doc: git-add: clarify explanation of editing file

Match the tone of this paragraph to the earlier paragraphs and continue
with the same example.

Author: jvns

Documentation/git-add.adoc

@@ -29,10 +29,10 @@ to commit your changes to that file, you can run:
 
 You can also add only part of your changes to a file with `git add -p`.
 
-This command can be performed multiple times before a commit.  It only
-adds the content of the specified file(s) at the time the add command is
-run; if you want subsequent changes included in the next commit, then
-you must run `git add` again to add the new content to the index.
+The `git add` command only stages the changes at the time that you run
+it. If you edit `file.c` after running `git add file.c`, you need to
+stage the new changes before committing, for example by running
+`git add file.c` again.
 
 The `git status` command can be used to obtain a summary of which
 files have changes that are staged for the next commit.