doc: Mention environment variable ZSTD in the TAP tests for MSVC
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