Document gen_types

https://github.com/holoviz/param/pull/966 added some internal utility to allow running `isinstance/issubclass` checks against a generator that yields types. This is useful for avoiding importing libraries on import, for example. @hoxbro used the same code in HoloViews and added a comment to use the code from Param when HoloViews' pin on Param gets bumped to the version that added `gen_types`. This function was not documented, but if we decide to use it elsewhere, it becomes de facto public API and we need to document it (at least add it to the API reference). If not, we should make it clearly part of Param's private API.

cc https://github.com/holoviz/holoviews/pull/6476#discussion_r2075271194