doc: Better explain which complex result is chosen by .^ * expr.txi: Explain which complex root is chosen and how to choose a real one if preferred. * data.cc (power): Ditto. * help.cc (.^, .**): Ditto. (^, **): Suggest using real-valued functions.

Copyright notices are intentionally omitted from the files in the
examples directory.  We don't want copyright notices cluttering the
manual.