json: docs - more details about the result of json_get_field()

- reported by GH #1542

(cherry picked from commit 6a64100105e7ac0ae8a952a2e3770d7534615796)
(cherry picked from commit 5041f3483ee9d579e144f77f5e43e1e406121e8a)

src/modules/json/doc/json_admin.xml

@@ -63,6 +63,12 @@
 	    <para>
 		Copy field 'field_name' from json object 'json_string' and store it in pvar 'destination'.
 		</para>
+		<para>
+		Note that the result is the string representation of that JSON filed.
+		For example, if you want to retrieve a field that has a string value,
+		then the result is the value enclosed in quotes. If the field value is a structure,
+		then the result is the string-formatted representation of that structure.
+		</para>
 		<example>
 		<title><function>json_get_field</function> usage</title>
 		<programlisting format="linespecific">