mk/compiler.mk: Adjust USE_LANGUAGES documentation to recent changes

Setting dialects in USE_LANGUAGES has been deprecated in favor of
USE_CXX_FEATURES and FORCE_CXX_STD (which is a major improvement in
cleanliness of expression).  Actually say this, rather than
documenting that it is expected to use USE_LANGUAGES for the FORCE
case.

This is a comment-only change.

Proposed in concept on tech-pkg with no objections.

(gdt)