Add insertion order note to keyset documentation

[kostmo]

Please complete the information below:

Where is the problem?

Keyset documentation:
https://docs.hhvm.com/hack/arrays-and-collections/vec-keyset-and-dict#keyset

What is the problem?

It should be noted that keysets preserve insertion order just like dict.
The dict documentation includes the note:

dicts are ordered according to the insertion order.

This note should be replicated for keysets, as in:

A keyset can only contain string or int values. keysets are ordered according to the insertion order.

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Please don't change anything below this point.

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• Build ID: HHVM=HHVM-4.164.0:HSL=v4.108.1:2023-06-17T02:05:59+0000:1fa47f258c6b68f8ec01899aa82fd6ffa0957109
• Page requested: /hack/arrays-and-collections/vec-keyset-and-dict
• Page requested at: Sat, 17 Jun 2023 02:59:15 +0000
• Controller: GuidePageController