FIX: add documentation about .json_symtab generation to the tests

(@tautschnig)

Author: hannes-steffenhagen-diffblue

regression/symtab2gb/multiple_symtabs/entry_point.adb

@@ -0,0 +1,7 @@
+with User;
+with Library;
+procedure Entry_Point is
+begin
+  User;
+  Library (-5);
+end Entry_Point;