Rustdoc syntax fixes

- Fix mismatched code blocks
- Remove newline from link (revealed by rust-lang/rust#44229)

src/base.rs

@@ -411,11 +411,11 @@ impl AllUsers {
     /// `passwd` entries, are called in a thread-safe manner.
     ///
     /// These functions [modify a global
-    /// state](http://man7.org/linux/man-pages/man3/getpwent.3.html#
-    /// ATTRIBUTES), and if any are used at the same time, the state could
-    /// be reset, resulting in a data race. We cannot even place it behind
-    /// an internal `Mutex`, as there is nothing stopping another `extern`
-    /// function definition from calling it!
+    /// state](http://man7.org/linux/man-pages/man3/getpwent.3.html#ATTRIBUTES),
+    /// and if any are used at the same time, the state could be reset,
+    /// resulting in a data race. We cannot even place it behind an internal
+    /// `Mutex`, as there is nothing stopping another `extern` function
+    /// definition from calling it!
     ///
     /// So to iterate all users, construct the iterator inside an `unsafe`
     /// block, then make sure to not make a new instance of it until

src/cache.rs

@@ -24,7 +24,7 @@
 //! value. In this case, switching to `&mut self` would only allow for one user
 //! to be read at a time!
 //!
-//! ``norun
+//! ```norun
 //! let mut cache = UsersCache::empty_cache();
 //! let uid   = cache.get_current_uid();                     // OK...
 //! let user  = cache.get_user_by_uid(uid).unwrap()          // OK...
@@ -43,7 +43,7 @@
 //! we’re just trying the same trick as earlier. A simplified implementation of
 //! a user cache lookup would look something like this:
 //!
-//! ``norun
+//! ```norun
 //! fn get_user_by_uid(&self, uid: uid_t) -> Option<&User> {
 //!     let users = self.users.borrow_mut();
 //!     users.get(uid)