guide: Add details to Basics section of SymPy's Architecture
guide: Add details to Basics section of SymPy's Architecture

Add details explaining what goes on behind the syntactic sugar for
expressions. Adapted from part of the discussion from mailing list
(https://groups.google.com/forum/#!topic/sympy/4Pt1V322RHw/discussion).

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy