Better document BN_ULONG (in the DESCRIPTION near BN_num_bits_word(3))

Better document BN_ULONG (in the DESCRIPTION near BN_num_bits_word(3))
and BN_BITS2 (below RETURN VALUES).

While here, perform major reordering and rewriting
for precision and readability, in particular:
- Avoid misleading wordings like "size of a BIGNUM".
- Drop the trivial example.
- Move the pointers to RSA_size(3) and friends to CAVEATS.
- Stop recommending 8*BN_num_bytes() in this context because it is wrong, too.