doc: Add standard Environment section to pg_amcheck ref page

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/flat/8458c9c5-18f1-46d7-94c4-1c30e4f44908%40eisentraut.org

doc/src/sgml/ref/pg_amcheck.sgml

@@ -644,6 +644,24 @@ PostgreSQL documentation
   </para>
  </refsect1>
 
+ <refsect1>
+  <title>Environment</title>
+
+  <para>
+   <command>pg_amcheck</command>, like most other <productname>PostgreSQL</productname>
+   utilities,
+   also uses the environment variables supported by <application>libpq</application>
+   (see <xref linkend="libpq-envars"/>).
+  </para>
+
+  <para>
+   The environment variable <envar>PG_COLOR</envar> specifies whether to use
+   color in diagnostic messages. Possible values are
+   <literal>always</literal>, <literal>auto</literal> and
+   <literal>never</literal>.
+  </para>
+ </refsect1>
+
  <refsect1>
   <title>Notes</title>