allow selecting the backend by setting the environment variable MPL_BACKEND
added documentation on backend selection methods
minor fix for the documentation on backend selection methods (either of the methods -> any of the methods)
DOC : edits to copy of usage_faq.rst

Made how to set backend an ordered list
Added a warning about global use of MPLBACKEND or use of -dbackend
Added mplDeprecation warning when the -d argument is used. Minor improvements to the backend selection documentation.
Merge pull request #3710 from FlorianRhiem/master

allow selecting the backend by setting the environment variable MPLBACKEND