[Chapel Merge] Improve documentation around definition of subtype

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Branch: refs/heads/main
Revision: 6591b21
Author: mppf
Link: Unavailable
Log Message:

Merge pull request #19455 from mppf/docs-for-18798

Improve documentation around definition of subtype

Follow-up to PR #18798 and #15210.

This PR improves the language specification and docs for isSubtype to be
clearer about the definition of when one thing is a subtype of another.
It also adds more cross references.

Reviewed by @lydia-duncan - thanks!