Doc.: Operation string matching semantics not specified

[dsbos]

The page section currently at https://istio.io/latest/docs/reference/config/security/authorization-policy/#Operation doesn't specify the semantics of the strings in the various fields of Operation.

Although an example implies that "*" is a wildcard character, there is no normative (e.g., non-example) text that specifies exactly how the given string are interpreted.

(The documentation should be sufficient to answer questions such as:

• Are there any other wildcard characters?
• Is there an escape character (or any other construct) for literally matching a "*"?
• Can there be multiple uses of wildcarding in a given match string? (Some Istio uses of "*" in host names can appear only once.)
• What exactly are the semantics of "*"? Does it always just match zero to N arbitrary characters? Or does it do anything special when adjacent (or non-adjacent) to "/" or "." characters in a path- or host-match string?

)

[ericvn]

So changes needed around: https://github.com/istio/api/blob/master/security/v1beta1/authorization_policy.proto#L415