Merge branch 'tr/doc-tt'

Merge branch 'tr/doc-tt'

The documentation set has been updated so that literal commands,
configuration variables and environment variables are consistently
typeset in fixed-width font and bold in manpages.

* tr/doc-tt:
  doc: change configuration variables format
  doc: more consistency in environment variables format
  doc: change environment variables format
  doc: clearer rule about formatting literals