OctaveCodegen: doc updates, fix typos, clean up FIXMEs
OctaveCodegen: indenting rules use regex and else/elseif fixes

1.  Use regex-based rules instead of startswith, to be robust
    to variable names like "elsewhere" or "endless".  Add test
    based on those.

2.  "else" and "elseif" should both increase and decrease the
    indent level.  (Old code only worked by coincidence and would
    fail for variables starting with "e")