https: Unhide alt svc cache (#18101)

http: Unhide the AlternateProtocolCache configuration
and move some docs from http3_upstream.md to connection_pooling.rst

Risk Level: N/A - Docs only
Testing: N/A - Docs only
Docs Changes: Unhide the AlternateProtocolCache configuration and move some docs from http3_upstream.md to connection_pooling.rst
Release Notes: Added
Platform Specific Features: N/A

Signed-off-by: Ryan Hamilton <rch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ d7fdbbc567b5e3175240c1fa9827255275c53c36

envoy/extensions/filters/http/alternate_protocols_cache/v3/alternate_protocols_cache.proto

@@ -15,10 +15,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 
 // Configuration for the alternate protocols cache HTTP filter.
 // [#extension: envoy.filters.http.alternate_protocols_cache]
-// TODO(RyanTheOptimist): Move content from source/docs/http3_upstream.md to
-// docs/root/intro/arch_overview/upstream/connection_pooling.rst when unhiding the proto.
 message FilterConfig {
-  // [#not-implemented-hide:]
   // If set, causes the use of the alternate protocols cache, which is responsible for
   // parsing and caching HTTP Alt-Svc headers. This enables the use of HTTP/3 for upstream
   // servers that advertise supporting it.

envoy/extensions/upstreams/http/v3/http_protocol_options.proto

@@ -118,7 +118,6 @@ message HttpProtocolOptions {
     //   is alpha is not guaranteed to be API-stable.
     config.core.v3.Http3ProtocolOptions http3_protocol_options = 3;
 
-    // [#not-implemented-hide:]
     // The presence of alternate protocols cache options causes the use of the
     // alternate protocols cache, which is responsible for parsing and caching
     // HTTP Alt-Svc headers. This enables the use of HTTP/3 for origins that