gh-94673: Clarify About Runtime State Related to Static Builtin Types (gh-117761)

gh-94673: Clarify About Runtime State Related to Static Builtin Types (gh-117761)

Guido pointed out to me that some details about the per-interpreter state for the builtin types aren't especially clear.  I'm addressing that by:

* adding a comment explaining that state
* adding some asserts to point out the relationship between each index and the interp/global runtime state