docs: further notes on disabling plugin repo

[jthegedus]

Summary

This is a follow up to #1227 to document behaviours further as noted in #1227 (comment)

• note that plugin_repository_last_check_duration does not stop initial repo sync
• note that disable_plugin_short_name_repository does not remove the plugin repo if it has already been synced. Document how to remove the plugin repo if the user desires.
• consolidate the ~/.asdfrc example file contents under the repo root defaults file and inline file itself.

Other Information

[jthegedus]

It looks a bit messy explaining each option here and then in-lining this file in the docs where we then explain what each setting does in more detail. Seemed a link to the docs was better, assuming network and web access. I am on the fence and can be convinced on reversing this change.