clarify MIN_SIZE constraints in docs

[michaelkirk]

• [x] I agree to follow the project's code of conduct.

• [-] I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

Addendum to #93

Are these the docs that lead you astray in #92 @laundmo?

[laundmo]

If i understand #93 correctly the panic with a min-size of 1 is fixed. why does this require a docs change? the docs were clear before, just lacking the info that you couldn't use min-size 1.

[urschrei]

Because I added an additional check asserting that MIN_SIZE is greater than zero. This was implicit before, but with this change is both explicit and documented.

[michaelkirk]

bors r=rmanoka

[bors[bot]]

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