message: Add missing docs on `diagnostics_port::command::Error`
diff: @@ -12,8 +12,10 @@ use thiserror::Error;
  /// Failures that occur when trying to parse an incoming client message.
  #[derive(Debug, Error)]
  pub(super) enum Error {
  + /// Error processing a line using the shell-like lexer.
  #[error("failed to split line using shell lexing rules")]
  ShlexFailure,
  + /// Not a valid command input.
  #[error(transparent)]
  Invalid(#[from] structopt::clap::Error),
  }
  