update conversion docs to new format, revise arithmetic and colour

I added "optional arg" annotations as tips (not notes), and moved then to the end of the comment block:

• using tips makes them look different from seealso
• the first para of a doc comment is supposed to be a one-line summary, so we can't put optional args there, unfortunately

The Image class page now looks like:

So ifthenelse now has a sensible description, though icc_transform does not.