Improve 'switch off commons-logging' documentation

Update reference documentation to make it clearer that only
`spring-core` has a direct dependency on `commons-logging`.

Also reference the 'empty jar' alternative option as described in
the SLF4J FAQ.
Merge pull request #445 from snicoll/commons-logging-doc

* commons-logging-doc:
  Improve 'switch off commons-logging' documentation