add: Clarify documentation of -A and -u

add: Clarify documentation of -A and -u

The documentation of '-A' and '-u' is very confusing for someone who
doesn't already know what they do.  Describe them with fewer words and
clearer parallelism to each other and to the behavior of plain 'add'.

Also mention the default <pathspec> for '-A' as well as '-u', because
it applies to both.

Signed-off-by: Greg Price <redacted>
Signed-off-by: Junio C Hamano <redacted>