Backport of docs: clarify the requriment for cross-partition network into release/1.16.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #19052 to be assessed for backporting due to the inclusion of the label backport/1.16.

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Warning automatic cherry-pick of commits failed. If the first commit failed,
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The person who merged in the original PR is:
@huikang
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: unable to process merge commit: "ff2468543e54ea3819a859fe25382a44bfb972d1", automatic backport requires rebase workflow

The below text is copied from the body of the original PR.

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Description

As pointed by @hashi-derek , mesh-gateway has to be exported for cross-partition network. We don't have any tutorial on this use case, so put a note in the general introduction to Admin partition.

Testing & Reproduction steps

Links

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• 1ef51ad - ff24685

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR