docs: Add basic user-services documentation.

service-script-guide.md

@@ -509,3 +509,9 @@ This is a sensible default for daemons that are happy with `0.0.0.0`,
 but lets the user specify something else, like `rc_need="net.wan"` if
 he needs it. The burden is on the user to determine the appropriate
 service whenever he changes the daemon's configuration file.
+
+# User services
+
+User services are ran as the user that starts them. A few things to note
+then, are that command_user shouldn't be set, runtime data (such as pidfiles
+and sockets) should be placed in XDG_RUNTIME_DIR.

user-guide.md

@@ -176,3 +176,21 @@ OpenRC has wrappers for many common output tasks in libeinfo.
 This allows to print colour-coded status notices and other things.
 To make the output consistent the bundled service scripts all use ebegin/eend to 
 print nice messages.
+
+# User services
+
+OpenRC supports managing services for users.
+
+The init scripts are loaded from /etc/user.d/init.d, and
+${XDG_CONFIG_HOME}/openrc/init.d with ~/.config should XDG_CONFIG_HOME be unset.
+
+Configurations in ~/.config/openrc/conf.d should overwrite /etc/user.d/conf.d,
+and similarly options set in ~/.config/openrc/rc.conf overrides /etc/rc.conf.
+
+Runlevels are kept in ~/.config/openrc/runlevels.
+
+`openrc` and all `rc-*` tools provie a --user/-U flag to operate with user-mode
+services and runlevels.
+
+The XDG_RUNTIME_DIR variable must be set before calling openrc --user, as it's
+used to store state for openrc itself and the services it runs.