Add link to Rollback in transactions doc.

A common misconception with transaction rollback in Rails is that
exceptions will not be re-raised from nested transactions. This is only
true for ActiveRecord::Rollback. The documentation for that class makes
that clear.

A link to that class doc has been added, as well as an explicit
restatement that all other exceptions will be re-raised.

Author: dersam

activerecord/lib/active_record/transactions.rb

@@ -99,8 +99,9 @@ module Transactions
     # be propagated (after triggering the ROLLBACK), so you should be ready to
     # catch those in your application code.
     #
-    # One exception is the ActiveRecord::Rollback exception, which will trigger
-    # a ROLLBACK when raised, but not be re-raised by the transaction block.
+    # One exception is the {ActiveRecord::Rollback}[rdoc-ref:Rollback] exception, which will trigger
+    # a ROLLBACK when raised, but not be re-raised by the transaction block. Any
+    # other exception will be re-raised.
     #
     # *Warning*: one should not catch ActiveRecord::StatementInvalid exceptions
     # inside a transaction block. ActiveRecord::StatementInvalid exceptions indicate that an