[TASK] EXT:form - add missing documentation for select element options

Add documentation for select elements "prependOptionLabel" and
"prependOptionValue" options.

Resolves: #81797
Releases: master, 8.7
Change-Id: I0fd1d3ccbe7e2a2cbcbb08c7c30d81376353cda3
Reviewed-on: https://review.typo3.org/53406


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>