[native] Update docs for VARCHAR type length value

The documentation does not clearly indicate what the length value
in the varchar type definition means. Clear up that it is the number of
characters and not the number of bytes.

presto-docs/src/main/sphinx/language/types.rst

@@ -28,7 +28,7 @@ Integer
 
 .. note::
 
-    Cast conversion functions allow leading and trailing spaces when casting 
+    Cast conversion functions allow leading and trailing spaces when casting
     a string to ``TINYINT``, ``SMALLINT``, ``INTEGER``, or ``BIGINT``.
 
 ``TINYINT``
@@ -103,7 +103,7 @@ Example literals: ``DECIMAL '10.3'``, ``DECIMAL '1234567890'``, ``1.1``
 .. note::
 
     For compatibility reasons decimal literals without explicit type specifier (e.g. ``1.2``)
-    are treated as values of the ``DOUBLE`` type by default up to version 0.198. 
+    are treated as values of the ``DOUBLE`` type by default up to version 0.198.
     After 0.198 they are parsed as DECIMAL.
 
     - System wide property: ``parse-decimal-literals-as-double``
@@ -115,10 +115,14 @@ String
 ``VARCHAR``
 ^^^^^^^^^^^
 
-Variable length character data with an optional maximum length.
+Variable length character data with an optional maximum length indicating
+the maximum number of characters allowed.
 
 Example type definitions: ``varchar``, ``varchar(20)``.
 
+For ``varchar(20)`` the length value indicates that the string data may
+consist of up to 20 characters.
+
 SQL supports simple and Unicode string literals:
  - Literal string : ``'Hello winter !'``
  - Unicode string with default escape character: ``U&'Hello winter \2603 !'``
@@ -144,10 +148,10 @@ equal, but comparison of such values implicitly converts the types to the same
 length and pads with spaces so that the following query returns `true`:
 
 ``SELECT cast('example' AS char(20)) = cast('example    ' AS char(25));``
-    
+
 As with `VARCHAR`, a single quote in a `CHAR`
 literal can be escaped with another single quote:
-    
+
 ``SELECT CHAR 'All right, Mr. DeMille, I''m ready for my close-up.'``