Do not print true, false options for flags

The user cannot write `--list false` for a flag.

Fixes ConnorGray#19.

docs/examples/complex-app.md

@@ -47,9 +47,6 @@ does testing things
 
 * `-l`, `--list` — lists test values
 
-  Possible values: `true`, `false`
-
-
 
 
 <hr/>

src/lib.rs

@@ -416,7 +416,9 @@ fn write_arg_markdown(buffer: &mut String, arg: &clap::Arg) -> fmt::Result {
         .filter(|pv| !pv.is_hide_set())
         .collect();
 
-    if !possible_values.is_empty() {
+    if !matches!(arg.get_action(), clap::ArgAction::SetTrue)
+        && !possible_values.is_empty()
+    {
         let any_have_help: bool =
             possible_values.iter().any(|pv| pv.get_help().is_some());