nghttpx: Document configrevision API

doc/nghttpx.h2r

@@ -559,6 +559,9 @@ status
 code
   HTTP status code
 
+Additionally, depending on the API endpoint, ``data`` key may be
+present, and its value contains the API endpoint specific data.
+
 We wrote "normally", since nghttpx may return ordinal HTML response in
 some cases where the error has occurred before reaching API endpoint
 (e.g., header field is too large).
@@ -590,6 +593,23 @@ The one limitation is that only numeric IP address is allowd in
 is used while non numeric hostname is allowed in command-line or
 configuration file is read using :option:`--conf`.
 
+GET /api/v1beta1/configrevision
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This API returns configuration revision of the current nghttpx.  The
+configuration revision is opaque string, and it changes after each
+reloading by SIGHUP.  With this API, an external application knows
+that whether nghttpx has finished reloading its configuration by
+comparing the configuration revisions between before and after
+reloading.
+
+This API returns response including ``data`` key.  Its value is JSON
+object, and it contains at least the following key:
+
+configRevision
+  The configuration revision of the current nghttpx
+
+
 SEE ALSO
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