Allow for unused doc comments on macros

With Rust 1.35 we get compile errors due to doc comments that are added
to macro invocations but not actually included in the expanded output.
The rustc wrongly assumes that we want to document the resulting code
and not just provide details about the invocation itself.
This change explicitly allows for those cases. Alternatively we could
have "downgraded" the doc comments to normal comments or removed them
altogether. There is little difference between those alternatives.

nitrocli/src/args.rs

@@ -101,6 +101,7 @@ impl<'io> Stdio for ExecCtx<'io> {
 }
 
 /// The available Nitrokey models.
+#[allow(unused_doc_comments)]
 Enum! {DeviceModel, [
   Pro => "pro",
   Storage => "storage"
@@ -116,6 +117,7 @@ impl From<DeviceModel> for nitrokey::Model {
 }
 
 /// A top-level command for nitrocli.
+#[allow(unused_doc_comments)]
 Enum! {Command, [
   Config => ("config", config),
   Lock => ("lock", lock),

nitrocli/src/pinentry.rs

@@ -30,6 +30,7 @@ type CowStr = borrow::Cow<'static, str>;
 ///
 /// The available PIN types correspond to the PIN types used by the Nitrokey devices:  user and
 /// admin.
+#[allow(unused_doc_comments)]
 Enum! {PinType, [
   Admin => "admin",
   User => "user"