Improve 'switch off commons-logging' documentation
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.

spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework