MDL-14679 tidy phpdocs for sql_substr() a bit

lib/dml/moodle_database.php

@@ -1752,7 +1752,7 @@ abstract class moodle_database {
     }
 
     /**
-     * Returns the proper substr() function for each DB.
+     * Returns the proper substr() SQL text used to extract substrings from DB
      * NOTE: this was originally returning only function name
      *
      * @param string $expr some string field, no aggregates

lib/dml/mssql_native_moodle_database.php

@@ -1139,7 +1139,7 @@ class mssql_native_moodle_database extends moodle_database {
     }
 
     /**
-     * Returns the proper substr() function for each DB.
+     * Returns the proper substr() SQL text used to extract substrings from DB
      * NOTE: this was originally returning only function name
      *
      * @param string $expr some string field, no aggregates