- "Acquire MUTEX for the dynamic scope of BODY, setting it to VALUE or
-some suitable default value if NIL. If WAIT-P is non-NIL and the mutex
-is in use, sleep until it is available"
+ "Acquire MUTEX for the dynamic scope of BODY. If WAIT-P is true (the default),
+and the MUTEX is not immediately available, sleep until it is available.
+
+If TIMEOUT is given, it specifies a relative timeout, in seconds, on how long
+the system should try to acquire the lock in the contested case.
+
+If the mutex isn't acquired succesfully due to either WAIT-P or TIMEOUT, the
+body is not executed, and WITH-MUTEX returns NIL.
+
+Otherwise body is executed with the mutex held by current thread, and
+WITH-MUTEX returns the values of BODY.
+
+Historically WITH-MUTEX also accepted a VALUE argument, which when provided
+was used as the new owner of the mutex instead of the current thread. This is
+no longer supported: if VALUE is provided, it must be either NIL or the
+current thread."