Documentation: 'git add -A' can remove files

The current documentation fails to mention that 'git add -A/--all' can
remove files as well as add them, and it also does not say anything about
filepatterns (whether they are allowed, mandatory, or optional). It is
also not clear what the similarities and differences to the -u option are.

Update the intro paragraph (as suggested by Junio, with some minor edits)
to make it clear that 'git add' is able to delete and to also cover the -p
option.

Reword the description of -u to make it clearer (based on Björn
Steinbrink's suggestion).

Simplify the description of -A by saying "Like -u" and then describe the
differences (based on the suggestions by Björn Steinbrink and Junio).

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -14,8 +14,12 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-This command adds the current content of new or modified files to the
-index, thus staging that content for inclusion in the next commit.
+This command updates the index using the current content found in
+the working tree, to prepare the content staged for the next commit.
+It typically adds the current content of existing paths as a whole,
+but with some options it can also be used to add content with
+only part of the changes made to the working tree files applied, or
+remove paths that do not exist in the working tree anymore.
 
 The "index" holds a snapshot of the content of the working tree, and it
 is this snapshot that is taken as the contents of the next commit.  Thus
@@ -92,20 +96,23 @@ apply.
 
 -u::
 --update::
-	Update only files that git already knows about, staging modified
-	content for commit and marking deleted files for removal. This
-	is similar
-	to what "git commit -a" does in preparation for making a commit,
-	except that the update is limited to paths specified on the
-	command line. If no paths are specified, all tracked files in the
-	current directory and its subdirectories are updated.
+	Only match <filepattern> against already tracked files in
+	the index rather than the working tree. That means that it
+	will never stage new files, but that it will stage modified
+	new contents of tracked files and that it will remove files
+	from the index if the corresponding files in the working tree
+	have been removed.
++
+If no <filepattern> is given, default to "."; in other words,
+update all tracked files in the current directory and its
+subdirectories.
 
 -A::
 --all::
-	Update files that git already knows about (same as '\--update')
-	and add all untracked files that are not ignored by '.gitignore'
-	mechanism.
-
+	Like `-u`, but match <filepattern> against files in the
+	working tree in addition to the index. That means that it
+	will find new files as well as staging modified content and
+	removing files that are no longer in the working tree.
 
 -N::
 --intent-to-add::