Document the fact, that openFileBlocking can consume an OS thread indefinitely.

Also state that a deadlock can happen with the non-threaded runtime.
[ci skip]

libraries/base/GHC/IO/Handle/FD.hs

@@ -150,6 +150,11 @@ openFile fp im =
 -- This can be useful for opening a FIFO for writing: if we open in
 -- non-blocking mode then the open will fail if there are no readers,
 -- whereas a blocking open will block until a reader appear.
+-- 
+-- Note: when blocking happens, an OS thread becomes tied up with the
+-- processing, so the program must have at least another OS thread if
+-- it wants to unblock itself. By corollary, a non-threaded runtime
+-- will need a process-external trigger in order to become unblocked.
 --
 -- @since 4.4.0.0
 openFileBlocking :: FilePath -> IOMode -> IO Handle