completion: improve doc for complex aliases

completion: improve doc for complex aliases

The completion code can be told to use a particular completion for
aliases that shell out by using ': git <cmd> ;' as the first command of
the alias. This only works if <cmd> and the semicolon are separated by a
space, since if the space is missing __git_aliased_command returns (for
example) 'checkout;' instead of just 'checkout', and then
__git_complete_command fails to find a completion for 'checkout;'.

The examples have that space but it's not clear if it's just for
style or if it's mandatory. Explicitly mention it.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>