Add overview for RealModule documentation.

Author: stephentyrone

Sources/RealModule/Documentation.docc/RealModule.md

@@ -1,6 +1,28 @@
 # ``RealModule``
 
-A module that extends the Swift standard library protocols to provide
-additional operations for floating-point types.
+Extensions on the Swift standard library that provide functionality for
+floating-point types.
 
 ## Overview
+
+``RealModule`` provides four protocols that extend the standard library's
+numeric protocol hierarchy: AlgebraicField, ElementaryFunctions,
+RealFunctions, and Real.
+
+Types conforming to AlgebraicField represent
+[fields](https://en.wikipedia.org/wiki/Field_(mathematics)). These are the
+mathematical structures that typically form the elements of vectors and
+matrices, so this protocol is appropriate for writing generic code to do
+linear-algebra-type operations.
+
+ElementaryFunctions provides bindings for the "math functions": the logarithm
+and exponential functions, sine, cosine and tangent as well as their inverses,
+and other functions that you may be familiar with from trigonometry and
+calculus. RealFunctions refines ElementaryFunctions and provides functions that
+are primarily used with the real numbers, such as atan2, erf and gamma, and
+the base-2 and -10 logarithm and exponential funtions.
+
+The Real protocol is a convenient name for the intersection of `FloatingPoint`,
+`RealFunctions`, and `AlgebraicField`; this is the protocol that you are most
+likely to want to constrain to when writing generic "math" code that works
+with floating-point types.