Fix doc nits

Many tiny changes to stdlib doc comments to make them consistent (for example
"Returns foo", rather than "Return foo", per RFC1574), adding missing periods, paragraph
breaks, backticks for monospace style, and other minor nits.

https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text

library/std/src/sys/sync/mutex/itron.rs

@@ -13,7 +13,7 @@ pub struct Mutex {
     mtx: SpinIdOnceCell<()>,
 }
 
-/// Create a mutex object. This function never panics.
+/// Creates a mutex object. This function never panics.
 fn new_mtx() -> Result<abi::ID, ItronError> {
     ItronError::err_if_negative(unsafe {
         abi::acre_mtx(&abi::T_CMTX {
@@ -31,7 +31,7 @@ impl Mutex {
         Mutex { mtx: SpinIdOnceCell::new() }
     }
 
-    /// Get the inner mutex's ID, which is lazily created.
+    /// Gets the inner mutex's ID, which is lazily created.
     fn raw(&self) -> abi::ID {
         match self.mtx.get_or_try_init(|| new_mtx().map(|id| (id, ()))) {
             Ok((id, ())) => id,

library/std/src/sys/sync/rwlock/futex.rs

@@ -222,7 +222,7 @@ impl RwLock {
         }
     }
 
-    /// Wake up waiting threads after unlocking.
+    /// Wakes up waiting threads after unlocking.
     ///
     /// If both are waiting, this will wake up only one writer, but will fall
     /// back to waking up readers if there was no writer to wake up.

library/std/src/sys/sync/rwlock/queue.rs

@@ -186,7 +186,7 @@ struct Node {
 }
 
 impl Node {
-    /// Create a new queue node.
+    /// Creates a new queue node.
     fn new(write: bool) -> Node {
         Node {
             next: AtomicLink::new(None),

library/std/src/sys/sync/rwlock/solid.rs

@@ -28,7 +28,7 @@ impl RwLock {
         RwLock { rwl: SpinIdOnceCell::new() }
     }
 
-    /// Get the inner mutex's ID, which is lazily created.
+    /// Gets the inner mutex's ID, which is lazily created.
     fn raw(&self) -> abi::ID {
         match self.rwl.get_or_try_init(|| new_rwl().map(|id| (id, ()))) {
             Ok((id, ())) => id,

library/std/src/sys/sync/thread_parking/futex.rs

@@ -36,7 +36,7 @@ pub struct Parker {
 // Ordering::Release when writing NOTIFIED (the 'token') in unpark(), and using
 // Ordering::Acquire when checking for this state in park().
 impl Parker {
-    /// Construct the futex parker. The UNIX parker implementation
+    /// Constructs the futex parker. The UNIX parker implementation
     /// requires this to happen in-place.
     pub unsafe fn new_in_place(parker: *mut Parker) {
         unsafe { parker.write(Self { state: Atomic::new(EMPTY) }) };

library/std/src/sys/sync/thread_parking/id.rs

@@ -30,7 +30,7 @@ impl Parker {
         Parker { state: AtomicI8::new(EMPTY), tid: UnsafeCell::new(None) }
     }
 
-    /// Create a new thread parker. UNIX requires this to happen in-place.
+    /// Creates a new thread parker. UNIX requires this to happen in-place.
     pub unsafe fn new_in_place(parker: *mut Parker) {
         parker.write(Parker::new())
     }

library/std/src/sys/sync/thread_parking/pthread.rs

@@ -92,7 +92,7 @@ pub struct Parker {
 }
 
 impl Parker {
-    /// Construct the UNIX parker in-place.
+    /// Constructs the UNIX parker in-place.
     ///
     /// # Safety
     /// The constructed parker must never be moved.

library/std/src/sys/sync/thread_parking/windows7.rs

@@ -95,7 +95,7 @@ const NOTIFIED: i8 = 1;
 // Ordering::Release when writing NOTIFIED (the 'token') in unpark(), and using
 // Ordering::Acquire when reading this state in park() after waking up.
 impl Parker {
-    /// Construct the Windows parker. The UNIX parker implementation
+    /// Constructs the Windows parker. The UNIX parker implementation
     /// requires this to happen in-place.
     pub unsafe fn new_in_place(parker: *mut Parker) {
         parker.write(Self { state: AtomicI8::new(EMPTY) });