[GRASS5] datum shifts

You wrote:

sorry to put another burden on you: to complete the proj update,
please also add notes in the related HTML pages ([mrsv].proj.html).

Done for [rsv].proj.html. I have not made changes to m.proj or
m.proj2.
I'll get to those as my time permits. Input for those modules are not
GRASS-specific data types. There are other tools to perform the same
function for generic data.

Roger