* Remove the mention of 'return true' being legal.
This is only allowed for compat with code from before MW 1.21,
when I added hook aborting as well as tolerance for null to
be the same as true.
Since then, there is no reason to return null or return true.
For early returns, an explicitly void 'return;' should be used
instead. Returning anything else like null/true serves no purpose
other than to confuse the reader.
* Mark the interfaces as natively 'void' with return type hints.
This means in core code that runs the hook, static analysis
like Phan and in IDEs, it will be known that these onFoo()s
never return a value, thus allowing them to detect if its
return value is assigned or used in a conditional for any
reason, which can be an easy mistake.
It also means that in the future when extensions start using
these interfaces in 'implements' statement, they will be
required to mark their hooks as void.
That migration is opt-in and still up ahead. This is not
a breaking change even for all existing extensions where
a return true/null may exist in an abortable hook since this
only applies to code directly typed against the interface.
The internal run() method doesn't care.
Change-Id: Ib79289bd486ac97cec492e72f9a8dee70cf2f6c2
01f3efe4ff