Add a note about Field/Method vs. TYPE_USE annotations in reference docs

[bclozel]

See this discussion in the spring-javaformat project. We already explain the bulk of it for collections, but it would help comparing with other annotations developers are used to.

[sdeleuze]

The intent was to mention it in that section:

A key difference is that Spring’s deprecated null-safety annotations, which follow JSR 305 semantics, apply to fields, parameters, and return values; while JSpecify annotations apply to type usage.
[...]

It is also recommended to move field and return value annotations closer to the type and on the same line, for example:

For fields, instead of @nullable private String field with Spring annotations, use private @nullable String field with JSpecify annotations.

For method return types, instead of @nullable public String method() with Spring annotations, use public @nullable String method() with JSpecify annotations.

I can maybe refine why "It is also recommended to move field and return value annotations closer to the type and on the same line" and mention explicitly @Target(ElementType.TYPE_USE) there?

[bclozel]

I see now that bit in the "Migrating from Spring null-safety annotations". I think the content is what we needed. I guess people should not approach this from a "Jakarta validation annotations vs. JSpecify annotations" in the first place, so I think this existing documentation does the job already.

[sdeleuze]

After a second thought, and after seeing #35123, I think there is room for improvement. It should not be required to see the migration documentation to have the information, and we can be a bit more explicit about it to make this information more visible.