[MINOR] document edge case of updateFunc usage

## What changes were proposed in this pull request?

Include documentation of the fact that the updateFunc is sometimes called with no new values. This is documented in the main documentation here: https://spark.apache.org/docs/latest/streaming-programming-guide.html#updatestatebykey-operation however from the docs included with the code it is not clear that this is the case.

## How was this patch tested?

PR only changes comments. Confirmed code still builds.

Author: Wil Selwood <wil.selwood@sa.catapult.org.uk>

Closes #18088 from wselwood/note-edge-case-in-docs.