If you would like to make a change, improvement or addition, it should be requested by opening a ticket in our bug tracker. If any relevant previous discussion has happened (on the mailing lists, forums, IRC or even edit notes), make sure to link that discussion to the ticket.
Make sure that the ticket is pretty clear on what is exactly being requested. While it's not necessary for the requester to write the final documentation, it's perfectly acceptable to submit a draft that makes the request more clear. If your proposed change is a non-trivial change, please include as much detail as possible in your ticket.
The Style Leader/BDFL (reosarevok) will review the request and decide how to proceed. Trivial and/or obviously useful additions are very likely to be accepted immediately. In the cases where the Style Leader feels more community input is needed, they'll request it, by calling for an informational vote and/or initiating further discussion. Please note that any vote called by the Style Leader will not be binding; the BDFL will not be required to act in accordance with the vote outcome.
Once enough input has been achieved for the Style Leader to make an informed decision, they will document the decision on the relevant ticket and then update the needed documentation/links.
Unreasonable requests, or requests that, while useful, would require unreasonable amounts of developer time that just won't be available to MusicBrainz anytime soon, will be rejected.
Updates to style issues will be posted regularly (every two weeks, as long as something has changed at all) on the MusicBrainz blog. The changes themselves might be made earlier than that, although major changes will generally happen in the same schedule as new code releases.