Add config option for executor process user

Also adds an 'executor' section to the configuration documentation.

Also fixes an attribute name typo.

Change-Id: I1f8387527e485d04ee3e22ba4673e9a0f02dd224
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 3721076..a78a0fa 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -169,6 +169,29 @@
   Path to PID lock file for the merger process.
   ``pidfile=/var/run/zuul-merger/merger.pid``
 
+executor
+""""""""
+
+The zuul-executor process configuration.
+
+**git_dir**
+  Directory that Zuul should clone local git repositories to.
+  ``git_dir=/var/lib/zuul/git``
+
+**log_config**
+  Path to log config file for the executor process.
+  ``log_config=/etc/zuul/logging.yaml``
+
+**private_key_file**
+  SSH private key file to be used when logging into worker nodes.
+  ``private_key_file=~/.ssh/id_rsa``
+
+**user**
+  User ID for the zuul-executor process. In normal operation as a daemon,
+  the executor should be started as the ``root`` user, but it will drop
+  privileges to this user during startup.
+  ``user=zuul``
+
 .. _connection:
 
 connection ArbitraryName