is-scalar.xml Improve description (#4013)

As all functions this is not limited to variables.

Author: mmalferov

reference/var/functions/is-scalar.xml

@@ -15,7 +15,8 @@
    <methodparam><type>mixed</type><parameter>value</parameter></methodparam>
   </methodsynopsis>
   <para>
-   Finds whether the given variable is a scalar.
+   Finds whether an <link linkend="language.expressions">expression</link> is
+   evaluated as a scalar value.
   </para>
   <para>
    See <link linkend="language.types.type-system.atomic.scalar">scalar types</link> for more information.