Document IPsiSourceFilePropertiesProvider to show how to change source file properties

[citizenmatt]

Decorate or replace IPsiSourceFileProperties to e.g. disable code model or building of PSIs.

Also, document IProjectPsiModuleProviderFilter and IPsiModuleDecorator which allow for replacing/decorating the whole of IProjectPsiModuleHandler or just decorating module references and source files.

[citizenmatt]

E.g. https://devnet.jetbrains.com/message/5528315#5528315