Fix FAQ wording for permissionless harvests

[rossgalloway]

Summary

Fix the FAQ wording around permissionless harvests.

Problem

The current FAQ wording implies broader permissionless harvest support than is actually true.

Correct behavior

Only V2 Curve factory vaults have permissionless harvests.

Proposed changes

• update the FAQ/docs wording to reflect the actual scope
• review surrounding language for any similar overgeneralization

Reference

• https://docs.yearn.fi/getting-started/guides/user-faq

Tracker

• To-do ID: T-010