Some more doc suggestions

Just some notes based on reading the docs for the first time. Some of these may be more substantive suggestions than pure doc changes - no offense intended. The two that might be iffy are:

The existing note about nonces is a bit self-contradictory. Using a random buffer of data as a nonce contradicts the previous sentence ("never ever be reused with the same key"). I don't know enough Nacl to know if the nonce really needs to be a nonce, or is an IV. If it's more of an IV construction then changing the prior sentence may make more sense. If it's a real nonce, then adding partitionable sequences may be better. 

The second note like that is about sodium_memcmp() , I think it's worth having a cautionary note that it's not memcmp compatible, but would you consider having a sodium_equals() instead? 

Edit: This PR now includes a non-doc change; update mlock()/munlock() to call madvise ... DONTDUMP.