Document the pre_composition signal.

docs/signals.rst

@@ -17,12 +17,22 @@ will be called with:
 * ``source_file`` - The absolute path to the file containing front matter.
 * ``frontmatter`` - The front matter dictionary that was loaded.
 
+pre_composition
+----------------
+
+``pre_composition`` fires before processing the entire site. When the
+watcher is running (see :ref:`devserver`), the signal will fire before
+handling any file or directory change.
+Any handler function that connects to the signal will be called with:
+
+* ``director`` - The director instance that processed the site.
+
 post_composition
 ----------------
 
 ``post_composition`` fires after processing the entire site. When the
 watcher is running (see :ref:`devserver`), the signal will fire after
-any file or directory change. Any handler function that connects to the
-signal will be called with:
+handling any file or directory change.
+Any handler function that connects to the signal will be called with:
 
 * ``director`` - The director instance that processed the site.