springboot Clarify that configuring a custom health HTTP status mapping replaces the default mappings

Clarify that configuring a custom health HTTP status mapping replaces the default mappings. (#20334)

Fixes gh-20334

Comment From: norio-io

I tried to fix #20334 . Please check.

Comment From: wilkinsona

Thanks very much for making your first contribution to Spring Boot, @norio-io.