Improve documentation of test parts

Added documentation for the TEST_PART environment variable.
Fixed an example.  Changed the text below the example to be more
clear.
Add documentation for the "@TEST-IGNORE" keyword
Merge remote-tracking branch 'origin/master' into topic/dnthayer/doc-improvements
Improve documentation of btest sphinx functionality

Mentioned that a ".btest" file extension is added to test files.
Mentioned that btest-rst-cmd and btest-rst-pipe check the exit code
of the specified command line argument.
Explained how the btest-rst-include and "btest-rst-cmd -r" commands
locate the specified file.
Added documentation of the "-n" option to btest-rst-cmd and btest-rst-include.
More improvements to btest sphinx documentation

Changed Finalizer to PartFinalizer.  Fixed an example.
Switched the order of a few sentences to make more sense.
Improve documentation of btest-include directive
Fix the btest-rst-pipe script
Merge remote-tracking branch 'origin/topic/dnthayer/doc-improvements'

* origin/topic/dnthayer/doc-improvements:
  Fix the btest-rst-pipe script
  Improve documentation of btest-include directive
  More improvements to btest sphinx documentation
  Improve documentation of btest sphinx functionality
  Add documentation for the "@TEST-IGNORE" keyword
  Improve documentation of test parts

BIT-1080 #merged