Currently incompatible changes are not clearly outlined in the related
documentations, such as interface-changes.rst. The impacts and suggested
actions to address the compatibility concerns also rarely exist.

This adds a requirement to include these information for incompatible
changes, so that such changes are clearly conveyed to the users, with
suggested actions to minimize the impacts of such changes.

Currently there is no policy on what should be done when an immediate,
severe breakage happens for "important/often used parts", which can cause
serious concerns especially if such changes remain obscured.

This adds a provision for this kind of incompatible changes under
"extraordinary circumstances", which allows for the breaking change to happen
if there is a critical reason to do so. Such changes must be extensively
documented, and should be avoided as much as possible.

Documented default values are relied upon by scripts, so changing these
values can casuse incompatible changes. This makes documenting them
required.

The default values for --ao, --vo, --gpu-api, --gpu-context change from
time to time, but some libmpv clients rely on the existing autoprobe order
and breaks as a result when the value changes. This specifies that the
default autoprobe order should not be relied upon.

As an instance of "extraordinary circumstances", such incompatibility
must be extensively documented.