Hide %A placeholder with ? if attr not found.

When using %A?(not_found_attr), this placeholder is hidden
from output. Example:

$ git log -1 --format="%h %s %A(not_found)"
93864e3 Commit message. %A(not_found)
$ git log -1 --format="%h %s %A?(not_found)"
93864e3 Commit message.
$
Adds documentation for pretty formats (%A).