Improve documentation in subclasses of `InjectExtension`

[mechite]

You cannot provides io.avaje.inject.spi.ConfigPropertyPlugin with an @ServiceProvider
implementation. The error message:

Missing `provides io.avaje.inject.spi.InjectExtension with ...DConfigPropertyPlugin`

@SentryMan I think we faced some similar issue before, but for a different one of these
InjectExtension subtypes?

(you can build just fine if you provides the basic InjectExtension, and my instinct says
it works fine at runtime due to spi instanceof check in DServiceLoader)

[mechite]

OK, I think this is an issue in documentation.
InjectPlugin says:

Add extensions by implementing one of the following extension interfaces and have it
registered with the ServiceLoader as an InjectExtension

AvajeModule, ConfigPropertyPlugin, InjectPlugin and maybe even ModuleOrdering
need to all specify an @see InjectPlugin.

...but I think this looks like intentional behaviour?

[SentryMan]

Yeah intentional

[SentryMan]

it will not work if you don't register as inject extension

[mechite]

Documentation of ConfigPropertyPlugin specifically almost implies that one should
provides it. so adding an @see is a good idea

[mechite]

@SentryMan we both did the same thing ^ - yours can take precedence