Explain a little better what BN_num_bits() and BN_num_bits_word() do.

Add a note as to how these functions do not always return the key size, and
how one can deal with that.

PR: 907

doc/crypto/BN_num_bytes.pod

@@ -16,8 +16,14 @@ BN_num_bits, BN_num_bytes, BN_num_bits_word - get BIGNUM size
 
 =head1 DESCRIPTION
 
-These functions return the size of a B<BIGNUM> in bytes or bits,
-and the size of an unsigned integer in bits.
+BN_num_bytes() returns the size of a B<BIGNUM> in bytes.
+
+BN_num_bits_word() returns the number of significant bits in a word.
+If we take 0x00000432 as an example, it returns 11, not 16, not 32.
+Basically, except for a zero, it returns I<floor(log2(w))+1>.
+
+BN_num_bits() returns the number of significant bits in a B<BIGNUM>,
+following the same principle as BN_num_bits_word().
 
 BN_num_bytes() is a macro.
 
@@ -25,9 +31,23 @@ BN_num_bytes() is a macro.
 
 The size.
 
+=head1 NOTES
+
+Some have tried using BN_num_bits() on individual numbers in RSA keys,
+DH keys and DSA keys, and found that they don't always come up with
+the number of bits they expected (something like 512, 1024, 2048,
+...).  This is because generating a number with some specific number
+of bits doesn't always set the highest bits, thereby making the number
+of I<significant> bits a little lower.  If you want to know the "key
+size" of such a key, either use functions like RSA_size(), DH_size()
+and DSA_size(), or use BN_num_bytes() and multiply with 8 (although
+there's no real guarantee that will match the "key size", just a lot
+more probability).
+
 =head1 SEE ALSO
 
-L<bn(3)|bn(3)>
+L<bn(3)|bn(3)>, L<DH_size(3)|DH_size(3)>, L<DSA_size(3)|DSA_size(3)>,
+L<RSA_size(3)|RSA_size(3)>
 
 =head1 HISTORY