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

doc/src/sgml/install-windows.sgml

@@ -534,9 +534,9 @@ $ENV{PROVE_TESTS}='t/020*.pl t/010*.pl'
     <varlistentry>
      <term><varname>GZIP_PROGRAM</varname></term>
      <listitem><para>
-       Path to a <application>gzip</application> command. The default is
-       <literal>gzip</literal>, that would be the command found in
-      <varname>PATH</varname>.
+      Path to a <application>gzip</application> command. The default is
+      <literal>gzip</literal>, which will search for a command by that
+      name in the configured <envar>PATH</envar>.
      </para></listitem>
     </varlistentry>
 
@@ -544,8 +544,8 @@ $ENV{PROVE_TESTS}='t/020*.pl t/010*.pl'
      <term><varname>LZ4</varname></term>
      <listitem><para>
       Path to a <application>lz4</application> command. The default is
-      <literal>lz4</literal>, that would be the command found in
-      <varname>PATH</varname>.
+      <literal>lz4</literal>, which will search for a command by that
+      name in the configured <envar>PATH</envar>.
      </para></listitem>
     </varlistentry>
 
@@ -553,8 +553,17 @@ $ENV{PROVE_TESTS}='t/020*.pl t/010*.pl'
      <term><varname>TAR</varname></term>
      <listitem><para>
       Path to a <application>tar</application> command. The default is
-      <literal>tar</literal>, that would be the command found in
-      <varname>PATH</varname>.
+      <literal>tar</literal>, which will search for a command by that
+      name in the configured <envar>PATH</envar>.
+     </para></listitem>
+    </varlistentry>
+
+    <varlistentry>
+     <term><varname>ZSTD</varname></term>
+     <listitem><para>
+      Path to a <application>zstd</application> command. The default is
+      <literal>zstd</literal>, which will search for a command by that
+      name in the configured <envar>PATH</envar>.
      </para></listitem>
     </varlistentry>
    </variablelist>