Allow ARel SQL Literal Nodes For Limit

The v3.0.4 limit regression and security fix here [1] should allow for ARel's SQL literal values to pass thru sanitization intact. This patch just adds a few lines to allow that while changing the documentation a bit. This patch also adds the SQLServerAdapter to the list of adapters that should ignore comma seperated values for limit strings.

I guess it could be argued that anyone really wanting to put comma separated values as a limit can no just pass Arel.sql strings which would simplify this method. But I did not want to presume an implementation change to force people to do that. If deemed appropriate, I can resubmit another patch that officially removes support from that method for said parsing and this whole thing can be a lot simplier.

[1] https://github.com/rails/rails/commit/354da43ab0a10b3b7b3f9cb0619aa...