Rollup merge of #37141 - nabeelomer:master, r=sfackler

Documented that RwLock might panic

Fixes https://github.com/rust-lang/rust/issues/37010

src/libstd/sync/rwlock.rs

@@ -136,6 +136,10 @@ impl<T: ?Sized> RwLock<T> {
     /// This function will return an error if the RwLock is poisoned. An RwLock
     /// is poisoned whenever a writer panics while holding an exclusive lock.
     /// The failure will occur immediately after the lock has been acquired.
+    ///
+    /// # Panics
+    ///
+    /// This function might panic when called if the lock is already held by the current thread.
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn read(&self) -> LockResult<RwLockReadGuard<T>> {
@@ -188,6 +192,10 @@ pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<T>> {
     /// This function will return an error if the RwLock is poisoned. An RwLock
     /// is poisoned whenever a writer panics while holding an exclusive lock.
     /// An error will be returned when the lock is acquired.
+    ///
+    /// # Panics
+    ///
+    /// This function might panic when called if the lock is already held by the current thread.
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn write(&self) -> LockResult<RwLockWriteGuard<T>> {