Update include archived project in search documentation

What does this MR do and why?

Some context for this documentation update,

• Hide/Include archived project feature was first introduced in 16.1 only for project scope, then it was expanded to issues scope in 16.3.
• In 16.6, the feature is available to all the scopes in the search result page where it applies. The FFs for all the scopes are removed by now, except [Feature flag] Rollout of `search_project_wikis... (#420152 - closed) which is expected to be enabled by default in the next two days.
• Because it's in all the scopes now, it feels redundant to mention it for any single scope. So, I am updating the docs to make it more generic.

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Related to #429920 (closed)