doc: add flags section to document all flags

Adds a section at the very end to document all flags and links
to it from every function that takes a flag argument.

Fixes: #16971

PR-URL: #20042
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: indranil