Relax promises about condition variable.

This allows for futex or thread parking based implementations in the
future.

library/std/src/sync/condvar.rs

@@ -78,13 +78,9 @@ impl WaitTimeoutResult {
 /// and a mutex. The predicate is always verified inside of the mutex before
 /// determining that a thread must block.
 ///
-/// Functions in this module will block the current **thread** of execution and
-/// are bindings to system-provided condition variables where possible. Note
-/// that this module places one additional restriction over the system condition
-/// variables: each condvar can be used with precisely one mutex at runtime. Any
-/// attempt to use multiple mutexes on the same condition variable will result
-/// in a runtime panic. If this is not desired, then the unsafe primitives in
-/// `sys` do not have this restriction but may result in undefined behavior.
+/// Functions in this module will block the current **thread** of execution.
+/// Note that any attempt to use multiple mutexes on the same condition
+/// variable may result in a runtime panic.
 ///
 /// # Examples
 ///
@@ -159,10 +155,8 @@ impl Condvar {
     ///
     /// # Panics
     ///
-    /// This function will [`panic!`] if it is used with more than one mutex
-    /// over time. Each condition variable is dynamically bound to exactly one
-    /// mutex to ensure defined behavior across platforms. If this functionality
-    /// is not desired, then unsafe primitives in `sys` are provided.
+    /// This function may [`panic!`] if it is used with more than one mutex
+    /// over time.
     ///
     /// [`notify_one`]: Self::notify_one
     /// [`notify_all`]: Self::notify_all