man: document how to include an equals sign in a value provided to Environment= (#5710)

It wasn't clear before how an equals sign in an "Environment=" value might be
handled. Ref:
http://stackoverflow.com/questions/43278883/how-to-write-systemd-environment-variables-value-which-contains/43280157

man/systemd.exec.xml

@@ -352,7 +352,7 @@
         assignments have no effect. Variable expansion is not
         performed inside the strings, however, specifier expansion is
         possible. The $ character has no special meaning. If you need
-        to assign a value containing spaces to a variable, use double
+        to assign a value containing spaces or the equals sign to a variable, use double
         quotes (") for the assignment.</para>
 
         <para>Example: