Support rbs types

[skatkov]

See rdoc support for these types:

https://github.com/ruby/rdoc/pull/1665/changes#diff-887a5d0c2ce351cf24efc493789352c0746f43d0cb8c600e634deb584d4ae134

[skatkov]

Upstream connection note: directly related to ruby/rdoc#1665.

ruby/rdoc master at bc0baee787609f2205e8f1103f3e1d36b923184a still does not expose the RBS type-signature support from that PR (RDoc::RbsHelper was not available in the tested master bundle). Recheck this issue after ruby/rdoc#1665 merges, or test against that PR branch specifically.