* OSC: Clarify control messages are also sent as feedback
This is already mentioned on the feedback page, but mentioning it above
the list of control messages helps interpreting that list.
* OSC: Remove errant commas in addresses
The rest of the docs do not use commas to separate the address from
arguments, so remove them here. These commas seem to have been
introduced first in commit 1138f1d (OSC: update OSC docs.) and were
probably copy-pasted into docs added after that.
* OSC: Fix and complete plugin operations and feedback
This makes the documentation complete by adding missing messages,
clarifying some existind documenation and fixing `/select/plug_page`
(address and parameter description were incorrect).
