parsers: Only warn on missing docs

Erroring out was making us add /// TODO to avoid the error, and that is
much less visible than a warning.

parsers/src/lib.rs

@@ -21,7 +21,7 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-#![deny(missing_docs)]
+#![warn(missing_docs)]
 
 pub use crate::util::error::Error;
 pub use jid::{BareJid, FullJid, Jid, JidParseError};