Note that certain flags are documented

Note that certain flags are documented

This is useful in Devel::PPPort for generating its api-info data.  That
useful feature of D:P allows someone to find out what was the first
release of Perl to have a function, macro, or flag.  And whether using
ppport.h backports it further.

I went through apidoc.pod and looked for flags that were documented but
that D:P didn't know about.  This commit adds entries for each so that
D:P can find them.