More details around failure reasons. (#5)

* Add more details into specific failure reasons.

* remove spaces.

* Add Makefile.

* Fix tab in Makefile.

* Refine definitions.

Makefile

@@ -0,0 +1,8 @@
+VENV := build/venv
+DOCS_OUTPUT_DIR ?= generated/docs
+
+.PHONY: docs
+docs:
+	rm -fr generated/docs
+	mkdir -p generated/docs
+	docs/build.sh $(DOCS_OUTPUT_DIR)

docs/configuration/http_conn_man/access_log.rst

@@ -70,15 +70,15 @@ entry. The following command operators are supported:
 %FAILURE_REASON%
   Additional failure reason if any in addition to response code. Possible values are:
 
-  * **LH**: Local health check failed.
-  * **UH**: No healthy upstream.
-  * **UT**: Upstream request timeout.
-  * **LR**: Connection local reset.
-  * **UR**: Upstream remote reset.
-  * **UF**: Upstream connection failure.
-  * **UC**: Upstream connection termination.
-  * **UO**: Upstream overflow (circuit breaking).
-  * **NR**: No route configured for a given request.
+  * **LH**: Local service failed :ref:`health check request <arch_overview_health_checking>` in addition to 503 response code.
+  * **UH**: No healthy upstream hosts in upstream cluster in addition to 503 response code.
+  * **UT**: Upstream request timeout in addition to 504 response code.
+  * **LR**: Connection local reset in addition to 503 response code.
+  * **UR**: Upstream remote reset in addition to 503 response code.
+  * **UF**: Upstream connection failure in addition to 503 response code.
+  * **UC**: Upstream connection termination in addition to 503 response code.
+  * **UO**: Upstream overflow (:ref:`circuit breaking <arch_overview_circuit_break>`) in addition to 503 response code.
+  * **NR**: No :ref:`route configured <arch_overview_http_routing>` for a given request in addition to 404 response code.
 
 %UPSTREAM_HOST%
   Upstream host URL (e.g., tcp://ip:port for TCP connections).