change RETURN_VALUE_DISCARDED GDScript warn text

changed RETURN_VALUE_DISCARDED GDscript warning text to mention how the return value of a function is discarded; update GDScript parser warning test to include new warning text.

modules/gdscript/gdscript_warning.cpp

@@ -96,7 +96,7 @@ String GDScriptWarning::get_message() const {
 		} break;
 		case RETURN_VALUE_DISCARDED: {
 			CHECK_SYMBOLS(1);
-			return "The function '" + symbols[0] + "()' returns a value, but this value is never used.";
+			return "The function '" + symbols[0] + "()' returns a value that will be discarded if not used.";
 		} break;
 		case PROPERTY_USED_AS_FUNCTION: {
 			CHECK_SYMBOLS(2);

modules/gdscript/tests/scripts/parser/warnings/return_value_discarded.out

@@ -2,4 +2,4 @@ GDTEST_OK
 >> WARNING
 >> Line: 6
 >> RETURN_VALUE_DISCARDED
->> The function 'i_return_int()' returns a value, but this value is never used.
+>> The function 'i_return_int()' returns a value that will be discarded if not used.