Registry doc cleanup

[tomdonaldson]

A couple items to clean up in the registry rst doc as follow up to #289:

1. tables = resources[4].get_tables() should get a # doctest: +IGNORE_WARNINGS to ignore unrelated warnings during doctests.
2. There are 2 "all-VO" queries that appropriately have .. doctest-skip:: since they would take a long time and frequently fail. Since that code is for illustration purposes only, consider:
   • clarifying those limitations with a comment
   • don't skip the example, but modify it to limit its run time and frailty (e.g., limit it to a couple iterations only with a suitable comment on how it would look for all-VO)

[bsipocz]

The first point has been addressed in #358, and a separate issue #359 has been opened for it (sorry, I didn't see this one in time).

[msdemlei]

Since there's #359, closing this.