Use the correct tense of open and keep

Minor language fix: "it opens" ... "it keeps"
Add a note about non-memcmp compatibility

sodium_memcmp is really an equality tester, rather than a replacement
for memcmp, add a warning note for implementors.
Add a note about mlock() failure due to ulimits
Minor tense fix for set

"it sets" ...
Remove the recommendation to use random bytes for nonces

My reading of randombytes_buf() is that it generates a byte buffer from a
source of random data that is likely to be uniformly random.

Although it's highly likely that crypto_box_NONCEBYTES of data will be unique
for most applicaiton invokations, there doesn't appear to be any kind of
gaurantee; and how should the implementor reason about re-keying? When they've
sent 2 ^ (crypto_box_NONCEBYTES * 8) bytes, or before?

Would things be simpler and more clear with partitionable sequences?