Assorted updates to md5.1

Note that the -c argument's parameter is compared against the digest of
the file, not the file. [1]

Update the "current time" parentheticals for notes about reversing
and colliding the hash functions. [1]

Some general mdoc updates.

PR: docs/188043 [1]
Submitted by: Jamie Landeg-Jones [1]
Approved by: hrs (mentor)
MFC after: 1 week