`UnsafeSlice.to_str` frozen string pitfalls

[jscheid]

Now, it's quite obvious that UnsafeSlice is... unsafe, but to help the next surprised poor soul I suggest you add something like the following to the documentation of UnsafeSlice.to_str:

The returned string is frozen, which means that any slices, slices of slices, and even the result of .dup, .clone, String.new(str) (of the whole string or any slices) will still reference the underlying memory. To make a true copy that won't keep a reference:

String.new(capacity: str.bytesize).concat(str)

To copy a slice:

IO::Buffer.for(str).get_string(ofs, len)

[saulecabrera]

I believe it's reasonable to add that to the documentation. @jscheid if you're willing to contribute such change, I'm happy to review a PR.