[Bug sanitizer/64706] -fsanitize=vptr is not documented

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64706

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 34513
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34513&action=edit
gcc5-pr64706.patch

Possible patch.  Not sure about the wording.