Docs: mention key rejection as AES rotation trigger (re-auth thundering herd)

[adisivaprasad]

What does this PR do?

Documents that minion key rejection (salt-key -r) triggers AES key rotation — just like key removal (salt-key -d) — causing all minions to re-authenticate with the master.

Two documentation changes:

1. Performance guide (doc/topics/tutorials/intro_scale.rst, "Too many minions re-authing" section): now mentions removal/rejection and salt-key -r alongside salt-key -d, as suggested in the issue.
2. CLI reference (doc/ref/cli/salt-key.rst): adds a prominent warning that deleting/rejecting keys rotates the AES publication key, which can cause a "thundering herd" of minion re-auths on large installations, with a backlink to the performance guide section (new :ref: target too-many-minions-re-authing).

Note: the third location mentioned in the issue, the legacy en/getstarted/system/communication.html page ("Rotating security keys" section), no longer exists in this repository — the get-started docs were migrated to the salt-user-guide / salt-install-guide repos, whose security pages already describe key rotation. Happy to open a companion PR there if maintainers would like.

What issues does this PR fix or reference?

Fixes #63469

Previous Behavior

The performance guide only mentioned master restart and key removal (salt-key -d) as events that rotate the AES key; key rejection was not mentioned, and the salt-key reference carried no warning about the re-auth thundering-herd effect of key deletion/rejection.

New Behavior

Rejection is documented as a rotation trigger in both the performance guide and the salt-key CLI docs (with a warning and cross-link).

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated (docs-only change, N/A)

Commits signed with GPG?

No

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