docs: Correct facts around xDS ACK semantics

[tonya11en]

trafficstars

An ACK does not mean that a config was successfully applied. Corrected this in the protocol documentation.

Risk Level: None. Doc change.

[repokitteh-read-only[bot]]

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:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/39891 was opened by tonya11en.

see: more, trace.

[repokitteh-read-only[bot]]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/). envoyproxy/api-shepherds assignee is @adisuissa CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/39891 was opened by tonya11en.

see: more, trace.

[botengyao]

/wait

Waiting for addressing @phlax and Adi's comments.