[release-1.7] clarify http retry attempts

[istio-testing]

Manual cherrypick required.

#1648 failed to apply on top of branch "release-1.7":

Applying: clarify retries attempts default
Applying: update doc
.git/rebase-apply/patch:91: trailing whitespace.
  // between retries will be determined automatically (25ms+). When request 
.git/rebase-apply/patch:92: trailing whitespace.
  // `timeout` of the [HTTP route](https://istio.io/docs/reference/config/networking/virtual-service/#HTTPRoute) 
.git/rebase-apply/patch:143: trailing whitespace.
  // between retries will be determined automatically (25ms+). When request 
.git/rebase-apply/patch:144: trailing whitespace.
  // `timeout` of the [HTTP route](https://istio.io/docs/reference/config/networking/virtual-service/#HTTPRoute) 
warning: 4 lines add whitespace errors.
Using index info to reconstruct a base tree...
M	kubernetes/customresourcedefinitions.gen.yaml
M	networking/v1alpha3/virtual_service.pb.go
M	networking/v1alpha3/virtual_service.pb.html
M	networking/v1alpha3/virtual_service.proto
M	networking/v1beta1/virtual_service.pb.go
M	networking/v1beta1/virtual_service.proto
Falling back to patching base and 3-way merge...
Auto-merging networking/v1beta1/virtual_service.proto
Auto-merging networking/v1beta1/virtual_service.pb.go
Auto-merging networking/v1alpha3/virtual_service.proto
Auto-merging networking/v1alpha3/virtual_service.pb.html
Auto-merging networking/v1alpha3/virtual_service.pb.go
Auto-merging kubernetes/customresourcedefinitions.gen.yaml
CONFLICT (content): Merge conflict in kubernetes/customresourcedefinitions.gen.yaml
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0002 update doc
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".