Edit to issues rate limits UI text and doc

What does this MR do?

Revised UI text for Admin Area > Settings > Network > Issues Rate Limits.

Also updated the accompanying docs screenshot to match.

Closes #330226 (closed)

Screenshots or Screencasts (strongly suggested)

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Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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