[Documentation]: Document transmute

[lydia-duncan]

Summary of Problem

Description:
This is related to #28188 and Damian's discourse post. The original issue was about example use cases, but I realized that we'd missed having documentation for the function at all due to it being placed in ChapelBase (which is documented piecemeal rather than wholesale). Since that seems like a higher priority (so users know about the feature at all) and potentially lower effort than a user's guide entry, it made sense to me to open a separate issue that could be closed independently.

[lydia-duncan]

Note that this is probably easier to accomplish and less development effort overall if the function gets moved to AutoMath first (see #28239, which talks about its stabilization). However, I don't want to gate the documentation on stabilizing the function, since stabilization conversations are exhausting.