Restore 'PreferencesForm' as alias for 'PreferencesFormLegacy'

Restore 'PreferencesForm' as alias for 'PreferencesFormLegacy'

In change 4633f4d46a8db46721474e9ffa6a130d8b7a2404 it was changed
to an interface implemented by both PreferencesFormLegacy and
PreferencesFormOOUI so that existing typehints for some functions
parameter would accept them both. Replace those typehints to use
HTMLForm instead. There was really no guarantee in the past that
they would only be given PreferencesForm or its subclasses, either.

Because the typehint change affects some hooks, note it as a
deprecation in MW 1.31 and a breaking change in MW 1.32.

Also add @since tags and correct some typos in code comments.

Follow-up to 4633f4d46a8db46721474e9ffa6a130d8b7a2404.

Change-Id: I61749f1d864cf68afe90cd9e15ba2d7a74252501