As of gh-24952, `PathPatternParser` will strictly reject patterns with
`"**"` in the middle of them. `"**"` is only allowed at the end of the
pattern for matching multiple path segments until the end of the path.
Currently, if `"**"` is used in the middle of a pattern it will be
considered as a single `"*"` instead. Rejecting such cases should
clarify the situation.
This commit prepares for that upcoming change and:
* logs a warning message if such a case is used by an application
* expands the MVC and WebFlux documentation about URI matching in
general
Closes gh-24958
dc4cda1b13