I found no documentation for the hidepid mount option (only applicable to procfs). I think the procfs manpage would be a good place to put this information. Unfortunately I did not find the info, so I have no patch. Maybe when I get in the mood to go through the kernel mailing list looking or the final version of the patch :)
There is some documentation in the kernel source file Documentation/filesystems/proc.txt, and I've reworked that text into the proc(5). Thanks for the report.