Enhanced docs of Pass Execution (If) keywords.
Enhanced docs of Pass Execution (If) keywords.

Update issue 174
Cc: pekka.klarck
Enhanced kw docs.

Also found some issues when looking at the code in BuiltIn:
- These keywords belong to _Control class.
- If-version should support non-existing variables in message and tags when
  condition is false.

userzimmermann/robotframework-python3,userzimmermann/robotframework-python3,userzimmermann/robotframework-python3,userzimmermann/robotframework-python3,userzimmermann/robotframework-python3