Improve the intro man page

Improve the intro man page

1. This adds the crypto_memcmp and crypto_zerocmp functions to the index
   and the SEE ALSO section.
2. SEE ALSO got reordered from mirroring the topical index to an
   alphabetical index.  Having the same list twice is to nobody's
   benefit and makes searching alphabetically (e.g. when the reader
   already has a vague idea what the function is supposed to be called)
   more difficult.