API: Update main description to reflect reality

API: Update main description to reflect reality

For quite a while now, the API no longer "may change at any time". Any
potential breaking change goes through code review, usage analysis, and
usually, a deprecation process.

This process was implemented as part of the "API roadmap"[1] RfC in
2014.

Today, the API is mature and stable, but still actively developed and
maintained. Breaking changes can still happen, and we want people to
subscribe to the -announce mailing list, but it's not as scary as the
old text implied.

[1]
<https://www.mediawiki.org/wiki/Requests_for_comment/API_roadmap#Deprecation_process>

Change-Id: If13172a35a3d041bce745aa72a835bfddcfe62c0