Make clear when the options min and max are required

Author: Bruno Vitorino

reference/constraints/Length.rst

@@ -168,9 +168,11 @@ max
 
 **type**: ``integer``
 
-This required option is the "max" length value. Validation will fail if
+This option is the "max" length value. Validation will fail if
 the given value's length is **greater** than this max value.
 
+This option is required when the ``min`` option is not defined.
+
 maxMessage
 ~~~~~~~~~~
 
@@ -193,9 +195,11 @@ min
 
 **type**: ``integer``
 
-This required option is the "min" length value. Validation will fail if
+This option is the "min" length value. Validation will fail if
 the given value's length is **less** than this min value.
 
+This option is required when the ``max`` option is not defined.
+
 It is important to notice that NULL values and empty strings are considered
 valid no matter if the constraint required a minimum length. Validators
 are triggered only if the value is not blank.