C++/Docs: expand on VN/HC and add wikipedia links

Author: Robert Marsh

docs/language/learn-ql/cpp/value-numbering-hash-cons.rst

@@ -4,7 +4,9 @@ Overview
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 In C and C++ QL databases, each node in the abstract syntax tree is represented by a separate object. This allows both analysis and results display to refer to specific appearances of a piece of syntax. However, it is frequently useful to determine whether two expressions are equivalent, either syntactically or semantically.
 
-The hash consing library (defined in ``semmle.code.cpp.valuenumbering.HashCons``) provides a mechanism for identifying expressions that have the same syntactic structure. The value numbering library (defined in ``semmle.code.cpp.valuenumbering.GlobalValueNumbering``) provides a mechanism for identifying expressions that compute the same value at runtime.
+The `hash consing <https://en.wikipedia.org/wiki/Hash_consing>`__ library (defined in ``semmle.code.cpp.valuenumbering.HashCons``) provides a mechanism for identifying expressions that have the same syntactic structure. The `global value numbering <https://en.wikipedia.org/wiki/Value_numbering>`__ library (defined in ``semmle.code.cpp.valuenumbering.GlobalValueNumbering``) provides a mechanism for identifying expressions that compute the same value at runtime.
+
+Both libraries divide the expressions in each function into equivalence classes represented by objects. Each ``HashCons`` object represents a set of expressions with identical parse trees, while ``GVN`` objects represent sets of expressions that will always compute the same value.
 
 
 Example C code