Document the generators for all the MODP groups

Saves the trouble of looking them up. Every group in RFC 3526 has
generator 2.

Change-Id: I58ec06510a6ca22a3bce03186e6eb7a510c7948e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/81427
Commit-Queue: Lily Chen <chlily@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Lily Chen <chlily@google.com>

include/openssl/dh.h

diff --git a/include/openssl/dh.h b/include/openssl/dh.h
index 4397d0e..4a3fb98 100644
--- a/include/openssl/dh.h
+++ b/include/openssl/dh.h
@@ -111,32 +111,38 @@
 
 // BN_get_rfc3526_prime_1536 sets |*ret| to the 1536-bit MODP group from RFC
 // 3526 and returns |ret|. If |ret| is NULL then a fresh |BIGNUM| is allocated
-// and returned. It returns NULL on allocation failure.
+// and returned. It returns NULL on allocation failure. The generator for this
+// group is 2.
 OPENSSL_EXPORT BIGNUM *BN_get_rfc3526_prime_1536(BIGNUM *ret);
 
 // BN_get_rfc3526_prime_2048 sets |*ret| to the 2048-bit MODP group from RFC
 // 3526 and returns |ret|. If |ret| is NULL then a fresh |BIGNUM| is allocated
-// and returned. It returns NULL on allocation failure.
+// and returned. It returns NULL on allocation failure. The generator for this
+// group is 2.
 OPENSSL_EXPORT BIGNUM *BN_get_rfc3526_prime_2048(BIGNUM *ret);
 
 // BN_get_rfc3526_prime_3072 sets |*ret| to the 3072-bit MODP group from RFC
 // 3526 and returns |ret|. If |ret| is NULL then a fresh |BIGNUM| is allocated
-// and returned. It returns NULL on allocation failure.
+// and returned. It returns NULL on allocation failure. The generator for this
+// group is 2.
 OPENSSL_EXPORT BIGNUM *BN_get_rfc3526_prime_3072(BIGNUM *ret);
 
 // BN_get_rfc3526_prime_4096 sets |*ret| to the 4096-bit MODP group from RFC
 // 3526 and returns |ret|. If |ret| is NULL then a fresh |BIGNUM| is allocated
-// and returned. It returns NULL on allocation failure.
+// and returned. It returns NULL on allocation failure. The generator for this
+// group is 2.
 OPENSSL_EXPORT BIGNUM *BN_get_rfc3526_prime_4096(BIGNUM *ret);
 
 // BN_get_rfc3526_prime_6144 sets |*ret| to the 6144-bit MODP group from RFC
 // 3526 and returns |ret|. If |ret| is NULL then a fresh |BIGNUM| is allocated
-// and returned. It returns NULL on allocation failure.
+// and returned. It returns NULL on allocation failure. The generator for this
+// group is 2.
 OPENSSL_EXPORT BIGNUM *BN_get_rfc3526_prime_6144(BIGNUM *ret);
 
 // BN_get_rfc3526_prime_8192 sets |*ret| to the 8192-bit MODP group from RFC
 // 3526 and returns |ret|. If |ret| is NULL then a fresh |BIGNUM| is allocated
-// and returned. It returns NULL on allocation failure.
+// and returned. It returns NULL on allocation failure. The generator for this
+// group is 2.
 OPENSSL_EXPORT BIGNUM *BN_get_rfc3526_prime_8192(BIGNUM *ret);