Resolve mandoc -Tlint nits

Resolve mandoc -Tlint nits

Several of them I have ignored intentionally:

1. unknown manual section comes with the definition;
2. unusual Xr order seems to have been an intentional decision
   grouping by topic rather than alphabetically,
   even if admittedly unusual;
3. no blank before trailing delimiter: Fa extern const
   crypto_argon2_extras crypto_argon2_no_extras;
   is another intentional choice,
   with no reasonable markup existing for a global variable
   declaration other than Bd.