website: document service maintenance endpoint

website/source/docs/agent/http.html.markdown

@@ -247,6 +247,7 @@ The following endpoints are supported:
 * [`/v1/agent/check/fail/<checkID>`](#agent_check_fail) : Mark a local test as critical
 * [`/v1/agent/service/register`](#agent_service_register) : Registers a new local service
 * [`/v1/agent/service/deregister/<serviceID>`](#agent_service_deregister) : Deregister a local service
+* [`/v1/agent/service/maintenance/<serviceID>`](#agent_service_maintenance) : Service maintenance mode
 
 ### <a name="agent_checks"></a> /v1/agent/checks
 
@@ -548,6 +549,20 @@ check, that is also deregistered.
 
 The return code is 200 on success.
 
+### <a name="agent_service_maintenance"></a> /v1/agent/service/maintenance/\<serviceId\>
+
+The service maintenance endpoint allows placing a given service into
+"maintenance mode". During maintenance mode, the service will be marked as
+unavailable, and will not be present in DNS or API queries.
+
+The `?enable` flag is required, and its value must be `true` (to enter
+maintenance mode), or `false` (to resume normal operation). It is an error to
+enable maintenance mode while it is already enabled, or disable it while it is
+already disabled. You will receive a 409 if either of these conflicts are
+observed.
+
+The return code is 200 on success.
+
 ## <a name="catalog"></a> Catalog
 
 The Catalog is the endpoint used to register and deregister nodes,