Fix concurrency docs

[RicHincapie]

Updates concurrency description at proxyConfig to match merged changes here istio/istio#43865

Resolves istio/istio.io#14777

[istio-policy-bot]

🤔 🐛 You appear to be fixing a bug in Go code, yet your PR doesn't include updates to any test files. Did you forget to add a test?

Courtesy of your friendly test nag.

[istio-policy-bot]

😊 Welcome @RicHincapie! This is either your first contribution to the Istio api repo, or it's been
a while since you've been here.

You can learn more about the Istio working groups, Code of Conduct, and contribution guidelines
by referring to Contributing to Istio.

Thanks for contributing!

Courtesy of your friendly welcome wagon.

[zirain]

please ran make gen locally

[istio-testing]

In response to a cherrypick label: #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".

[istio-testing]

In response to a cherrypick label: new issue created for failed cherrypick: #3141