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>