Cleanup clippy warnings

Enable clippy::missing_errors_doc

warning: docs for function returning `Result` missing `# Errors` section
   --> src/main.rs:164:5
    |
164 | /     pub fn run(mut self) -> io::Result<i32> {
165 | |         debug!("Running with options: {:#?}", self.options);
166 | |
167 | |         let mut files = Vec::new();
...   |
202 | |         self.print_dirs(dirs, no_files, is_only_dir, exit_status)
203 | |     }
    | |_____^
    |

src/main.rs

@@ -155,6 +155,9 @@ fn git_options(options: &Options, args: &[&OsStr]) -> Option<GitCache> {
 }
 
 impl<'args> Exa<'args> {
+    /// # Errors
+    ///
+    /// Will return `Err` if printing to stderr fails.
     pub fn run(mut self) -> io::Result<i32> {
         debug!("Running with options: {:#?}", self.options);