Documentation should cover plugin loss on upgrade

[simonw]

According to https://news.ycombinator.com/item?id=44110584#44111864

was running into this too until I started upgrading with

llm install -U llm

instead of

uv tool upgrade llm

[simonw]

I think the upgraded documentation should recommend using llm install -U llm and one people that they might have to reinstall their plugins otherwise.