enum: updated the docs in regards to max len for enum number

- synced with the code, following on a report from Alex Balashov

(cherry picked from commit 910be93)

modules/enum/README

@@ -72,7 +72,7 @@ Chapter 1. Admin Guide
    based on the user part of the current Request-URI. These functions
    assume that the Request URI user part consists of an international
    phone number of the form +decimal-digits, where the number of digits is
-   at least 2 and at most 15. Out of this number enum_query forms a domain
+   at least 2 and at most 20. Out of this number enum_query forms a domain
    name, where the digits are in reverse order and separated by dots
    followed by domain suffix that by default is "e164.arpa.". For example,
    if the user part is +35831234567, the domain name will be

modules/enum/doc/enum_admin.xml

@@ -21,7 +21,7 @@
 		based on the user part of the current Request-URI. These functions
 		assume that the Request URI user part consists of an international phone number 
 		of the form +decimal-digits, where the number of digits is at
-		least 2 and at most 15. Out of this number 
+		least 2 and at most 20. Out of this number
 		<function moreinfo="none">enum_query</function> forms a domain name, 
 		where the digits are in reverse order and separated by dots followed by 
 		domain suffix that by default is <quote>e164.arpa.</quote>. For example,