Add some comment

src/frontend/parse.c

@@ -4,6 +4,7 @@ Author: 1985 Wayne A. Christopher, U. C. Berkeley CAD Group
 **********/
 
 /*
+ * Control language parser:
  * A simple operator-precedence parser for algebraic expressions.
  * This also handles relational and logical expressions.
  */

src/spicelib/parser/inpptree.c

@@ -4,6 +4,8 @@ Author: 1987 Wayne A. Christopher, U. C. Berkeley CAD Group
 **********/
 //#define TRACE
 
+/* Tree generator for B-Source parser */
+
 #include "ngspice/ngspice.h"
 #include "ngspice/compatmode.h"
 #include "ngspice/ifsim.h"