
6c417bb has added the build infrastructure to support ZSTD, but forgot to update this section of the docs to mention the variable ZSTD, as per the change done in vcregress.pl. While on it, reword this section of the docs to describe what happens in the default case, as per a suggestion from Robert Haas. Discussion: https://postgr.es/m/YhCL0fKnDv/Zvtuo@paquier.xyz