[Chapel Merge] Document isNothingValue()

Branch: refs/heads/main
Revision: f43456b
Author: bradcray
Log Message:

Merge pull request #18401 from bradcray/doc-is-nothing-val

Document isNothingValue()

[trivial, not reviewed]

I think this one got overlooked in the recent effort to document
is*Value() routines due to the timing of the PR merges involved.

Modified Files:
M modules/standard/Types.chpl

Compare: https://github.com/chapel-lang/chapel/compare/65aff85a9eb1...f43456bf8623