As suggested by tb@, merge the description of OPENSSL_EC_NAMED_CURVE

As suggested by tb@, merge the description of OPENSSL_EC_NAMED_CURVE
and OPENSSL_EC_EXPLICIT_CURVE
from OpenSSL commit 146ca72c Feb 19 14:35:43 2015 +0000
after tb@ changed the default from 0 to OPENSSL_EC_NAMED_CURVE
in ec/ec_lib.c rev. 1.41,
which is the same default that OpenSSL uses since 1.1.0.

While merging, drop the description of the pre-1.1.0 behaviour.
It seems irrelevant to me because tb@ found no application in Debian
codesearch using OPENSSL_EC_EXPLICIT_CURVE.  A former devious default
that was probably never relied upon by anyone does not need to be
documented.