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

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bf4ed12b58205d8527053d53c8f473074e191b8d

Modified Files
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doc/src/sgml/install-windows.sgml | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)