[DOCS] Add note about sys_reaction table and TYPO3 11, references #278

Documentation/KnownProblems/Index.rst

@@ -6,8 +6,11 @@
 Known problems
 --------------
 
-There are no particular problems known currently, except for those limitations
-already mentioned in the documentation or reported in the bug tracker.
+In order to support Reactions in TYPO3 12, a new database field was added to the
+:code:`sys_reaction` table. Since this table does not exist in TYPO3 11, the Maintenance Tool
+will create an incomplete :code:`sys_reaction` table with just that single field.
+Since there's no associated TCA in TYPO3 11, this doesn't cause any problem. It's just
+not very elegant.
 
 In general please report bugs and improvements at:
 https://github.com/cobwebch/external_import/issues.