Warn about unsupported "/path/**/other" patterns with WebFlux PathPatternParser

[bclozel]

This issue is an early warning of behavior changes coming with #24952 in 5.3.0.

When applications use PathPatternParser to parse patterns with "**" in the middle, we should log a WARNING log message about this particular pattern saying that it will be processed as a "*" and rejected in the future.

We should also improve the Javadoc and reference documentation to better explain the cases supported here.