Update limitations of allow local network

[dledfordcf]

Limitations originally specifically stated Windows would only exclude a single subnet if the device had multiple. Confirmed this limitation is on all OS's

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Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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