Fix help text for generic flag to not insinuate that you can specify multiple

Feels like it may have been copied from StringSliceFlag or something,
but update the output to be more consistent with other single value
flags.

Also added comments to the String and Apply functions.

flag_test.go

@@ -262,15 +262,14 @@ var genericFlagTests = []struct {
 	value    cli.Generic
 	expected string
 }{
-	{"help", &Parser{}, "--help <nil>\t`-help option -help option` "},
-	{"h", &Parser{}, "-h <nil>\t`-h option -h option` "},
-	{"test", &Parser{}, "--test <nil>\t`-test option -test option` "},
+	{"test", &Parser{"abc", "def"}, "--test 'abc,def'\ttest flag"},
+	{"t", &Parser{"abc", "def"}, "-t 'abc,def'\ttest flag"},
 }
 
 func TestGenericFlagHelpOutput(t *testing.T) {
 
 	for _, test := range genericFlagTests {
-		flag := cli.GenericFlag{Name: test.name}
+		flag := cli.GenericFlag{Name: test.name, Value: test.value, Usage: "test flag"}
 		output := flag.String()
 
 		if output != test.expected {