Add nghttpx API section

doc/nghttpx.h2r

@@ -462,6 +462,42 @@ addresses:
 
     App.new
 
+API ENDPOINTS
+-------------
+
+nghttpx exposes API endpoints to manipulate it via HTTP based API.  By
+default, API endpoint is disabled.  To enable it, add a dedicated
+frontend for API using :option:`--frontend` option with "api"
+parameter.  All requests which come from this frontend address, will
+be treated as API request.
+
+The following section describes available API endpoints.
+
+PUT /api/v1beta/backend/replace
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This API replaces the current set of backend servers with the
+requested ones.  The request must carry request body with method PUT
+or POST.  The request body must be nghttpx configuration file format.
+For configuration file format, see `FILES`_ section.  The line
+separator inside the request body must be single LF (0x0A).
+Currently, only :option:`backend <--backend>` option is parsed, the
+others are simply ignored.  The semantics of this API is replace the
+current backend with the backend options in request body.  Describe
+the desired set of backend severs, and nghttpx makes it happen.  If
+there is no :option:`backend <--backend>` option is found in request
+body, the current set of backend is replaced with the :option:`backend
+<--backend>` option's default value, which is ``127.0.0.1,80``.
+
+The replacement is done instantly without breaking existing
+connections or requests.  It also avoids any process creation as is
+the case with hot swapping with signals.
+
+The one limitation is that only numeric IP address is allowd in
+:option:`backend <--backend>` in request body while non numeric
+hostname is allowed in command-line or configuration file is read
+using :option:`--conf`.
+
 SEE ALSO
 --------