fixed doc builder warnings; make sure the tests give reasonable results when OpenCL is not available

ReST directive was used. Also fixed some other ReST directives that were
not correct and removed some warnings during buildbot checks.

- Used lunix style slashes - works on most systems
- Removed all trailing whitespaces

All errors were found by doc/check_docs2.py

Documentation: fixed class/struc members documentation; added warning on incorrectly documented member

added documentation for ORB (ticket #1835). updated documentation on features2d (Feature2D class is still to be documented) (ticket #1819)

lot's of changes; nonfree & photo modules added; SIFT & SURF -> nonfree module; Inpainting -> photo; refactored features2d (ORB is still failing tests), optimized brute-force matcher and made it non-template.