[Chapel Merge] added doc for isXValue()

Branch: refs/heads/main
Revision: e9d99d1
Author: e-kayrakli
Log Message:

Merge pull request #17787 from strikeraryu/master

added doc for isXValue()

Documentation added for isXValue() as mentioned in Issue #17644.
Docs locally checked using make docs.

[Contributed by @strikeraryu, review/test/merge by @e-kayrakli]

Modified Files:
M modules/standard/Types.chpl

Compare: https://github.com/chapel-lang/chapel/compare/c0911e556eff...e9d99d101e96