Prior to this commit, the "Method Arguments" documentation for WebFlux
in the reference manual stated that WebFlux controller methods can
accept arguments of type Map, Model, or ModelMap to access the model.
However, ModelMap is actually not supported and results in exception
due to a type mismatch.
This commit updates the documentation to reflect this.
In addition, this commit updates related Javadoc and tests to avoid
mentioning or using ModelMap in WebFlux.
Closes gh-33107
8b11ee9ee2