The upstream project is officially named "LZ4", and the documentation
was confused with the option value that can be used with DDLs supporting
this option, and the project name.

Documentation related to the configure option --with-lz4 was missing, so
add something for that.

Author: Dilip Kumar, Michael Paquier
Reviewed-by: Justin Pryzby
Discussion: https://postgr.es/m/YJaOZQDXBVySq+Cc@paquier.xyz