Don't use an advice to override save-buffer

Using an advice for save-buffer might cause other Lisp programs to
fail. Instead we should override C-x C-s key binding, as it has already
been done for find-file (C-x C-f).

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>
Fix incorrect F3/F4/F6 information in help page

F3/F4/F6 key bindings always get the string at cursor position. They
used to get the selected text (if any), but it has been removed.

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>