* Update documentation for uniques
The documentation was outdated after merge of #11389
Using the widely spread term collections and item instead
of the previous class and instance.
* Update README.md
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Co-authored-by: Jegor Sidorenko <5252494+jsidorenko@users.noreply.github.com>
Co-authored-by: Jegor Sidorenko <jegor@parity.io>