Guide: Update reasons for members didn't arrive in time

- Timer resets on each chunk, member count should not be relevant anymore
- Discord introduced a rate limit to requesting all members, which could potentially cause this error - ref: https://discord.com/developers/docs/change-log/2025-08-14-introducing-guild-members-rate-limit

section: https://discordjs.guide/legacy/popular-topics/errors#members-didnt-arrive-in-time