Documentation: clarify multiple pushurls vs urls

Documentation: clarify multiple pushurls vs urls

In a remote with multiple configured URLs, `git remote -v` shows the
correct url that fetch uses. However, `git config remote.<remote>.url`
returns the last defined url instead. This discrepancy can cause
confusion for users with a remote defined as such, since any url
defined after the first essentially acts as a pushurl.

Add documentation to clarify how fetch interacts with multiple urls
and how push interacts with multiple pushurls and urls.

Add test affirming interaction between fetch and multiple urls.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>