[Azoth MuC] Added docs for the upcoming /setperm command.

Refs #1676, #1677, #1678, #1679, #1680, #1681, #1682, #1683, #1684, #1685.
[Azoth MuC] Added the /setperm command.

Closes #1676, #1677, #1678, #1679, #1680, #1681, #1682, #1683, #1684,
#1685, #1686.

This is slightly not the command that's been requested in those tickets,
but it's way more universal and protocol-agnostic.