[release-1.22] Update PeerAuthentication docs

[istio-testing]

Manual cherrypick required.

#3184 failed to apply on top of branch "release-1.22":

Applying: Update PeerAuthentication docs for mTLS
Applying: update
Applying: update text
Applying: made gen
Applying: make gen
Applying: fix gencheck
Using index info to reconstruct a base tree...
A	security/v1/peer_authentication_alias.gen.go
Falling back to patching base and 3-way merge...
CONFLICT (modify/delete): security/v1/peer_authentication_alias.gen.go deleted in HEAD and modified in fix gencheck. Version fix gencheck of security/v1/peer_authentication_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 0006 fix gencheck
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".

[craigbox]

Do we bother with cherrypicking to release-1.22 for API docs?