Summary:
Minor improvements to `Rcu.h`:
* Spell out the first use of RCU for the sake of people who are not familiar
  with the abbreviation.
* Correct the default template argument type in the comment: it's `RcuTag`, not
  `void`.
* Don't expose `rcu_token` constructor to the users because it's only supposed
  to be obtained via `lock_shared`.
* Make other single-argument constructors explicit to prevent undesirable
  conversions.
* Parameterize `rcu_token` on the tag type to prevent cross-domain use.

Reviewed By: yfeldblum

Differential Revision: D13510133

fbshipit-source-id: d5d214cfa3b30d0857c14ac293da6e4310db1100