document managing ruby patchlevels with alias

I'm not sure alias is the right place to document this since you'd need to know the solution to find it essentially, maybe it should go in troubleshooting? If there is a better place for it I'd be happy to move it.

Googling around for "can't switch ruby after rvm upgrade" and "specify rvm ruby patchlevel" didn't have great results. There is a StackOverflow question but this should probably be in the docs since they are normally the top 3 results for anything "rvm" related.