Move custom* filenames to private to be in sync with the type
PrivateRR: naming naming naming

Try to find better (=more in sync with the rest of the lib) naming. My
guess is that these are better, but YMMV.
remove private rr tests into example_test.go
Fix tests and examples.

Put all examples in the same file and cleanup the tests to use
the new names.
More naming and some docs
Small doc update