modules: readme files regenerated - modules ... [skip ci]

src/modules/json/README

@@ -77,6 +77,12 @@ Chapter 1. Admin Guide
    Copy field 'field_name' from json object 'json_string' and store it in
    pvar 'destination'.
 
+   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.
+
    Example 1.1. json_get_field usage
 ...
 json_get_field("{'foo':'bar'}", "foo", "$var(foo)");