Document the recently added branch commands (bug#50344)

Document the recently added branch commands (bug#50344)

* doc/emacs/maintaining.texi (VC Directory Commands): Change the
prefix key from "B" to "b".  Replace vc-create-tag with vc-create-branch,
and vc-retrieve-tag with vc-switch-branch.
(Switching Branches): Mention vc-switch-branch bound to 'C-x v b s'.
(Creating Branches): Mention vc-create-branch bound to 'C-x v b c'.

* lisp/vc/vc-dir.el (vc-dir-mode-map): In branch keymap 'b' rebind
"c" from vc-create-tag to vc-create-branch, and "s" from
vc-retrieve-tag to vc-switch-branch.