update alt_stat_name docs with cross references (#489)

Updated the docs as per the discussion at #484

Signed-off-by: Rama <rama.rao@salesforce.com>

docs/root/configuration/http_filters/router_filter.rst

@@ -150,8 +150,11 @@ x-envoy-upstream-alt-stat-name
 
 Setting this header on egress requests will cause Envoy to emit upstream response code/timing
 statistics to a dual stat tree. This can be useful for application level categories that Envoy
-doesn't know about. The output tree is documented :ref:`here
-<config_cluster_manager_cluster_stats_alt_tree>`.
+doesn't know about. The output tree is documented :ref:`here <config_cluster_manager_cluster_stats_alt_tree>`.
+
+This should not be confused with :ref:`alt_stat_name <envoy_api_field_Cluster.alt_stat_name>` which
+is specified while defining the cluster and when provided specifies an alternative name for the
+cluster at the root of the statistic tree.
 
 x-envoy-upstream-canary
 ^^^^^^^^^^^^^^^^^^^^^^^

envoy/api/v2/cds.proto

@@ -51,7 +51,9 @@ message Cluster {
   string name = 1 [(validate.rules).string.min_bytes = 1];
 
   // An optional alternative to the cluster name to be used while emitting stats.
-  // Any ``:`` in the name will be converted to ``_`` when emitting statistics.
+  // Any ``:`` in the name will be converted to ``_`` when emitting statistics. This should not be
+  // confused with :ref:`Router Filter Header
+  // <config_http_filters_router_x-envoy-upstream-alt-stat-name>`.
   string alt_stat_name = 28;
 
   // Refer to :ref:`service discovery type <arch_overview_service_discovery_types>`