request-pull: documentation updates

The original description talked only about what it does.  Instead,
start it with the purpose of the command, i.e. what it is used for,
and then mention what it does to achieve that goal.

Clarify what <start>, <url> and <end> means in the context of the
overall purpose of the command.

Describe the extended syntax of <end> parameter that is used when
the local branch name is different from the branch name at the
repository the changes are published.

Signed-off-by: Junio C Hamano <gitster@pobox.com>