Revert "Add documentation to TransactionIsolationError [ci skip]"

This reverts commit 5ff59704f6.

Reason: Seems dubious and adds less value.

[ci skip]

activerecord/lib/active_record/errors.rb

@@ -207,8 +207,6 @@ module ActiveRecord
   class ImmutableRelation < ActiveRecordError
   end
 
-  # Raised in database transaction blocks in situations described
-  # in the DatabaseStatements#transaction documentation
   class TransactionIsolationError < ActiveRecordError
   end
 end