Documentation tweaks for "Resource loader configuration"

[mwoodiupui]

1. It would be much less confusing to state explicitly that the value of bitronix.tm.resource.configuration is a filesystem path, not (as one might expect) a resource path, and so must be either an absolute path or expressed relative to the current directory of the JVM process. I had to search through the code to see why my attempts to set this were failing.

2. It should be stated that the properties shown under "Configuring a JDBC pool" are representative but not definitive, and that one must go look at the documentation for the configured datasource type to find out what properties are actually recognized. For example, as discussed some years ago on some mailing list that I can't identify from its remains on Nabble, PGXADataSource does not have a URL property...but it does have a Url property which worked when I tried it.