lingering USE_LANGUAGES documentation

[Greg Troxel <gdt%lexort.com@localhost>]

in mk/compiler.mk:

#       If the package assumes the compiler defaults to a specific
#       language version, the USE_LANGUAGES variable can also be used
#       to force the inclusion of flags such as -std=c++11. In this case,
#       USE_CC_FEATURES/USE_CXX_FEATURES should also be set.


My impression is that this is at best deprecated, and the proper
approach is FORCE_CXX_STD (which is documented just below the above).

And therefore, we should

  remove that paragraph

  from the list of valid values, drop the specific language families
  like c++11


If that's not right, then I would like to understand why, so we can
explain under what circumstances USE_LANGUAGES of e.g. c++11 is correct.