In dh.h rev. 1.22 2018/02/20 17:38:15, tb@ provided

In dh.h rev. 1.22 2018/02/20 17:38:15, tb@ provided
DH_clear_flags(3), DH_test_flags(3), and DH_set_flags(3),
in rev. 1.24 2018/02/20 18:01:42 DH_set_length(3), and
in rev. 1.25 2018/02/22 16:41:04, jsing@ provided DH_get0_engine(3).
Merge the documentation from OpenSSL, tweaked by me.