To be honest I don't know how much he has improved on the manual. It is such a small language that you could easily read up to the the examples very quickly even if you aren't particularly interested in learning.

I would suggest just giving it a look directly at https://www.gnu.org/software/sed/manual/sed.html

Though be forewarned, something that neither document explains well is the actual syntax. As in how addresses and expressions can be used and how to read a script. The syntax is relatively simple to understand looking at some examples, but the lack of clear delimiters between the address, command, and command parameters can confuse beginners.