Fix misleading Seq.init XML doc

[bbatsov]

Fixes #14233

The Seq.init XML doc claimed "Each element is saved after its initialization" but this is incorrect — Seq.init does not cache elements. The initializer function is reapplied on each iteration, exactly like Seq.initInfinite.

Changes:

• Updated the description to match actual behavior, using the same wording as Seq.initInfinite
• Added a <see> reference to Seq.cache for users who need cached results
• Removed the misleading "This is an O(n) operation" from the remarks (creating the sequence is O(1), same as initInfinite)

[github-actions]

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@bbatsov,

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