[release-1.22] Improve VS retry documentation

[istio-testing]

Manual cherrypick required.

#3247 failed to apply on top of branch "release-1.22":

Applying: Improve VS retry documentation
Applying: gen
Using index info to reconstruct a base tree...
A	networking/v1/virtual_service_alias.gen.go
M	networking/v1alpha3/virtual_service.pb.go
M	networking/v1alpha3/virtual_service.pb.html
A	networking/v1beta1/virtual_service_alias.gen.go
Falling back to patching base and 3-way merge...
CONFLICT (modify/delete): networking/v1beta1/virtual_service_alias.gen.go deleted in HEAD and modified in gen. Version gen of networking/v1beta1/virtual_service_alias.gen.go left in tree.
Auto-merging networking/v1alpha3/virtual_service.pb.html
Auto-merging networking/v1alpha3/virtual_service.pb.go
CONFLICT (modify/delete): networking/v1/virtual_service_alias.gen.go deleted in HEAD and modified in gen. Version gen of networking/v1/virtual_service_alias.gen.go left in tree.
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0002 gen
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".