Mark BN_mod_exp2_mont() as intentionally undocumented.

Mark BN_mod_exp2_mont() as intentionally undocumented.
It appears to be intended for internal use by DSA_do_verify(3) and using
codesearch.debian.net, i found nothing outside OpenSSL/LibreSSL using it.

In April 2018, jsing@ questioned whether the five related functions
BN_mod_exp_mont() and friends should even be exposed by <openssl/bn.h>,
so we decided to not document them.  Now tb@ agrees that there is no
reason to document BN_mod_exp2_mont() as long as we don't want to
document BN_mod_exp_mont().