mediawiki: make docs include plugin tags robuster for new MW version

mediawiki: make docs include plugin tags robuster for new MW version

Some (new) mediawiki versions can convert '<' and '>' to '&lt;' or
'&gt;', this resulted in a situation where our encoded tag did not
get detected by the decode logic, and thus a visitor only saw
something like the following:

> <--- @PVEDOCHACK@
> PCEtLSBhc2NpaWRvYyBIRUFERVIgLS0+DQo8c2NyaXB0IHR5.....
> @PVEDOCHACK@ -->

The word "HACK" was a bit unfortunate too, as it could be interpreted
as in the docs got hacked, by unknowing people. So change that too to
'PVEDOCS_BASE64'

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>