Do not have duplicate section heads

Do not have duplicate section heads

Change find-doc-nits to complain if a section header is repeated,
within a parent header (i.e., duplicate =head2 within a =head1).
In almost all cases, we just remove the duplicate header, as
it was a "continuation" of the =head1 that was already in affect.
In some cases, just remove "=head1 NOTES", possibly moving text
around, because the "NOTES" were really important parts of the
DESCRIPTION section.

No =headX sections should end with a period.
All =head1 labels should be in all uppercase.
No sub-head (=head2, etc) should be in all uppercase.
Update find-doc-nits to reject the above.

Fixup an internal POD link

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9631)