Add command socket support to zuul-merger

Like we have in zuul-executor, add command socket support for
zuul-merger.

Change-Id: I66a2cb2ba3f55bdd03e884f47648278e30d2f6ab
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/doc/source/admin/components.rst b/doc/source/admin/components.rst
index b3c2e44..d988351 100644
--- a/doc/source/admin/components.rst
+++ b/doc/source/admin/components.rst
@@ -282,6 +282,11 @@
 
 .. attr:: merger
 
+   ,, attr:: command_socket
+      :default: /var/lib/zuul/merger.socket
+
+      Path to command socket file for the merger process.
+
    .. attr:: git_dir
 
       Directory in which Zuul should clone git repositories.