Fix doc for EC_GROUP_set_curve()

Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9874)

doc/man3/EC_GROUP_new.pod

@@ -98,10 +98,12 @@ EC_GROUP_new() is the same as EC_GROUP_new_ex() except that the library context
 used is always the default library context.
 
 EC_GROUP_set_curve() sets the curve parameters B<p>, B<a> and B<b>. For a curve
-over Fp B<b> is the prime for the field. For a curve over F2^m B<p> represents
+over Fp B<p> is the prime for the field. For a curve over F2^m B<p> represents
 the irreducible polynomial - each bit represents a term in the polynomial.
 Therefore there will either be three or five bits set dependent on whether the
 polynomial is a trinomial or a pentanomial.
+In either case, B<a> and B<b> represents the coefficients a and b from the
+relevant equation introduced above.
 
 EC_group_get_curve() obtains the previously set curve parameters.