Update 'volatile-ttl' eviction policy description

[engpetarmarinov]

Clarified description of the 'volatile-ttl' eviction policy. Remove mentioned LFU, which is not correct.

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Note

Low Risk
Low risk doc-only change that clarifies eviction behavior; no code or runtime behavior is affected.

Overview
Updates the eviction policy docs to correct the description of volatile-ttl, removing the incorrect implication that it evicts based on LFU.

Applies the same clarification across the unversioned Redis Software docs and the versioned pages for 7.4, 7.8, and 7.22.

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[CLAassistant]

All committers have signed the CLA.

[rrelledge]

Thank you, @engpetarmarinov . Can you also please sign the CLA? #2859 (comment)