Consider deprecating *OptionList and moving them to the internal namespace

[coryan]

I am not sure why things like this belong in the public namespace:

google-cloud-cpp/google/cloud/bigtable/options.h

Lines 131 to 135 in eb1d0cc

	/// The complete list of options accepted by `bigtable::*Client`
	using ClientOptionList =
	OptionList<DataEndpointOption, AdminEndpointOption,
	InstanceAdminEndpointOption, MinConnectionRefreshOption,
	MaxConnectionRefreshOption>;

google-cloud-cpp/google/cloud/bigtable/options.h

Lines 163 to 165 in eb1d0cc

	using DataPolicyOptionList =
	OptionList<DataRetryPolicyOption, DataBackoffPolicyOption,
	IdempotentMutationPolicyOption>;

[coryan]

@dbolduc reminded me these are for discoverability. Being a "belt and suspenders" kind of person,I think we should expose these in the Doxygen documentation too.

[coryan]

We have improved the Doxygen pages to group options. Closing