(Doc) Clarify documentation for properties

These two pages:
https://logging.apache.org/log4j/2.0/manual/configuration.html#SystemProperties and http://logging.apache.org/log4j/2.x/manual/async.html#SysPropsMixedSync-Async
both talk about "system properties" only.

This should be amended to clarify that properties can also be specified by creating a file named log4j2.component.properties and include it in the classpath of the application.