Added reference link.

Co-authored-by: Craig Comstock <[email protected]>

lib/files.cf

@@ -1244,6 +1244,7 @@ body edit_field quoted_var(newval,method)
 # @brief Edit the quoted value of the matching line
 # @param newval The new value
 # @param method The method by which to edit the field (append|prepend|alphanum|set|delete)
+   see https://docs.cfengine.com/latest/reference-promise-types-files-edit_line-field_edits.html#field_operation
 {
       field_separator => "\"";
       select_field    => "2";