improve the doxygen documentation of the var-part of mupp.

src/musredit_qt6/mupp/var/include/PExpressionDef.hpp

@@ -8,6 +8,19 @@
   Based on Joel de Guzman example on calc7,
   see https://github.com/boostorg/spirit
 
+  This file contains the implementation (definition) of the PExpression
+  template grammar. It defines the actual grammar rules and their semantic
+  actions using Boost.Spirit Qi.
+
+  The grammar implements expression parsing with proper operator precedence:
+  - Primary expressions (literals, variables, functions)
+  - Unary operations (+, -)
+  - Multiplicative operations (*, /)
+  - Additive operations (+, -)
+
+  Symbol tables are populated with operators and function names, and error
+  handling is integrated for reporting parse failures.
+
 ***************************************************************************/
 
 /***************************************************************************