[release-1.21] Fix concurrency docs

[istio-testing]

Manual cherrypick required.

#3138 failed to apply on top of branch "release-1.21":

Applying: Fix concurrency docs
Using index info to reconstruct a base tree...
M	networking/v1beta1/proxy_config.pb.go
Falling back to patching base and 3-way merge...
Auto-merging networking/v1beta1/proxy_config.pb.go
CONFLICT (content): Merge conflict in networking/v1beta1/proxy_config.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 Fix concurrency docs
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".