Remove n=5 minimum from chia plots check and update documentation

[jon-lee]

chia plots check was a useful tool in pre-mainnet and early mainnet days as a way of validating new plotter implementations.
With the stability and proven track record of plotters, I think the documentation can be improved to remove suggestions that "bad plots" are those that underperform that can also be explained by statistical variance.

chia plots check is still a useful tool for identifying duplicates chia plots check -n 0 -l or for simply validating a plot file is readable. The latter can be accomplished with n=1, there's no need to enforce n=5 in my opinion.