DOC: Clarify that methods taking a MultiIndex level index also accept the name
DOC: further homogenized the description of "level" argument
DOC: level argument description in _binop
Merge pull request #6500 from toobaz/level_name

DOC: Clarify that methods taking a MultiIndex level index also accept the name
DOC: show users how to emulate R c function with iloc slicing and r_