[FEATURE] RTE: Cleanup in SpellCheckingController

Add comments. Clearer Aspell command construction.

Releases: master
Resolves: #62138
Change-Id: Ib553d782bc2602c53540e7a532f14cdd4e62fbb8
Reviewed-on: http://review.typo3.org/33241
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>