Document formatting recommendations for configuration property javadoc

[rhamedy]

Added a note to the spring-boot-features.adoc to clarify that formatting
configuration property descriptions is not supported.

Fixes gh-18572

[rhamedy]

Hi @philwebb
I added the NOTE suggested by @snicoll after the bullet point
Do not provide the default value in the description unless it has to be determined at runtime.
in the section suggested by @wilkinsona

Strangely enough, in my local environment (eclipse, using the provided installer) whenever I run ./mvnw clean install the nohttp-checkstyle-validation validation fails.

Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:3.0.0:check (nohttp-checkstyle-validation) on project spring-boot-build: You have 909 Checkstyle violations.

I wonder if I have to change some settings locally for the nohttp-checkstyle-validation. I have tested this PR's changes in an online tool https://asciidoclive.com and looks good 👍

[snicoll]

Thanks again @rhamedy. I've polished your contribution to use a single line per sentence. I've also moved the note outside of the section that describes what we do as this should be done regardless you're interested in our conventions or not.