Update documentation for CMath library
I updated description and 'Usage' part for
CMath library, added 'Brief overview of complex numbers' part and added two books about complex numbers. Also I added examples for each library method.
P.S.: this is my first redmine issue with patch, therefore if I did something wrong, please tell me about it.
merge revision(s) 50793,51120: [Backport #11162]
* lib/cmath.rb: [DOC] Add docs [ci skip][Fix GH-909][Bug #11162] Patch provided by @davydovanton