doc: clarify the behavior of uv_tty_init

Closes #120

PR-URL: https://github.com/libuv/libuv/pull/121
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

docs/src/tty.rst

@@ -58,8 +58,13 @@ API
     `readable`, specifies if you plan on calling :c:func:`uv_read_start` with
     this stream. stdin is readable, stdout is not.
 
+    On Unix this function will try to open ``/dev/tty`` and use it if the passed file
+    descriptor refers to a TTY. This lets libuv put the tty in non-blocking mode
+    without affecting other processes that share the tty.
+
     .. note::
-        TTY streams which are not readable have blocking writes.
+        If opening ``/dev/tty`` fails, libuv falls back to blocking writes for non-readable
+        TTY streams.
 
 .. c:function:: int uv_tty_set_mode(uv_tty_t*, uv_tty_mode_t mode)