clarify.

doc/crypto/DH_set_method.pod

@@ -56,7 +56,7 @@ the default method is used.
      /* compute shared secret */
         int (*compute_key)(unsigned char *key, BIGNUM *pub_key, DH *dh);
 
-     /* compute r = a ^ p mod m. May be NULL */
+     /* compute r = a ^ p mod m (May be NULL for some implementations) */
         int (*bn_mod_exp)(DH *dh, BIGNUM *r, BIGNUM *a, const BIGNUM *p,
                                 const BIGNUM *m, BN_CTX *ctx,
                                 BN_MONT_CTX *m_ctx);

doc/crypto/DSA_set_method.pod

@@ -62,12 +62,13 @@ struct
 	int (*dsa_do_verify)(const unsigned char *dgst, int dgst_len,
                                  DSA_SIG *sig, DSA *dsa);
 
-     /* compute rr = a1^p1 * a2^p2 mod m. May be NULL */
+     /* compute rr = a1^p1 * a2^p2 mod m (May be NULL for some
+                                          implementations) */
 	int (*dsa_mod_exp)(DSA *dsa, BIGNUM *rr, BIGNUM *a1, BIGNUM *p1,
                                  BIGNUM *a2, BIGNUM *p2, BIGNUM *m,
                                  BN_CTX *ctx, BN_MONT_CTX *in_mont);
 
-     /* compute r = a ^ p mod m. May be NULL */
+     /* compute r = a ^ p mod m (May be NULL for some implementations) */
         int (*bn_mod_exp)(DSA *dsa, BIGNUM *r, BIGNUM *a,
                                  const BIGNUM *p, const BIGNUM *m,
                                  BN_CTX *ctx, BN_MONT_CTX *m_ctx);

doc/crypto/RSA_set_method.pod

@@ -87,10 +87,11 @@ the default method is used.
 	int (*rsa_priv_dec)(int flen, unsigned char *from,
           unsigned char *to, RSA *rsa, int padding);
 
-     /* compute r0 = r0 ^ I mod rsa->n. May be NULL */
+     /* compute r0 = r0 ^ I mod rsa->n (May be NULL for some
+                                        implementations) */
 	int (*rsa_mod_exp)(BIGNUM *r0, BIGNUM *I, RSA *rsa);
 
-     /* compute r = a ^ p mod m. May be NULL */
+     /* compute r = a ^ p mod m (May be NULL for some implementations) */
 	int (*bn_mod_exp)(BIGNUM *r, BIGNUM *a, const BIGNUM *p,
           const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *m_ctx);
 

doc/crypto/rsa.pod

@@ -86,8 +86,9 @@ contain public as well as private RSA keys:
 In public keys, the private exponent and the related secret values are
 B<NULL>.
 
-B<dmp1>, B<dmq1> and B<iqmp> may be B<NULL> in private keys, but the
-RSA operations are much faster when these values are available.
+B<p>, B<q>, B<dmp1>, B<dmq1> and B<iqmp> may be B<NULL> in private
+keys, but the RSA operations are much faster when these values are
+available.
 
 =head1 CONFORMING TO