[Bug c/70967] doc: Please document rotate idiom

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70967

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I am not sure this is needed any more since there are now builtins for them:
__builtin_stdc_rotate_left 
__builtin_stdc_rotate_right