Some cleanup of L<> markup in pod files

Show only the #define, not the values, in BIO_f_buffer.  Data
abstraction and we can remove a "see also" entry.

Remove internal forward reference to NOTES in EVP_EncryptInit; just
say "see below" as we do in the other pages.

Add missing (3) in pem.pod so the L<> entry is consistent.
Fix entry to point to the "master" page, not the symlink'd one.

Reviewed-by: Matt Caswell <matt@openssl.org>