remote: use common "magit-remote-" prefix for commands

Rename `magit-rename-remote' to `magit-remote-rename,
rename `magit-add-remote to `magit-remote-add', and
rename `magit-remove-remote' to `magit-remote-remove'.
magit-remote-{*}: improve git usage info in doc-strings
Merge branch 'n/remote-commands' into next