This is a Homebrew tap with a few SGML-related packages. These are mainly intended for building the PostgreSQL documentation. Other than that, they are ancient, and you probably don't want them unless you know what you are doing.

Just brew tap petere/sgml and then brew install <formula>.

For the PostgreSQL documentation, you need:

brew install docbook docbook-dsssl docbook-sgml docbook-xsl openjade