Update outdated commands descriptions and cleanups in README (#11372)

Redis commands has been significantly refactored in 7.0.
This PR updates the outdated README.md to reflect it.

Based on #10864, doing some additional cleanups.

Co-authored-by: theoboldalex <theoboldalex@gmail.com>
Co-authored-by: Oran Agra <oran@redislabs.com>