[release-1.8] clarify INSERT support for HTTP_ROUTE

[istio-testing]

Manual cherrypick required.

#1625 failed to apply on top of branch "release-1.8":

Applying: clarify index support for HTTP_ROUTE
Using index info to reconstruct a base tree...
M	networking/v1alpha3/envoy_filter.pb.go
M	networking/v1alpha3/envoy_filter.pb.html
M	networking/v1alpha3/envoy_filter.proto
Falling back to patching base and 3-way merge...
Auto-merging networking/v1alpha3/envoy_filter.proto
CONFLICT (content): Merge conflict in networking/v1alpha3/envoy_filter.proto
Auto-merging networking/v1alpha3/envoy_filter.pb.html
CONFLICT (content): Merge conflict in networking/v1alpha3/envoy_filter.pb.html
Auto-merging networking/v1alpha3/envoy_filter.pb.go
CONFLICT (content): Merge conflict in networking/v1alpha3/envoy_filter.pb.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 clarify index support for HTTP_ROUTE
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

[GregHanson]

close by https://github.com/istio/api/pull/1748