Comment region BEG .. END.
CS and CE are the comment start string and comment end string,
respectively. CCS and CCE are the comment continuation strings
for the start and end of lines, respectively (default to CS and CE).
BLOCK indicates that end of lines should be marked with either CCE,
CE or CS (if CE is empty) and that those markers should be aligned.
LINES indicates that an extra lines will be used at the beginning
and end of the region for CE and CS.
INDENT indicates to put CS and CCS at the current indentation of
the region rather than at left margin.
(fn BEG END CS CE &optional CCS CCE BLOCK LINES INDENT)