Logback documentation explains how to initialize the logging system and
namely how the `logback.configurationFile` system property can be used to
specify the configuration file to use.

Spring Boot has an abstraction on top of that. A user can define the
`logging.path` property regardless of the logging infrastructure it is
using.

Users following the logback documentation can be confused at first so
we're not logging a warning when we found out that the logback specific
property has been specified.

Closes gh-2382