Make clear when the options min and max are required

Author: Bruno Vitorino

reference/constraints/Length.rst

@@ -122,9 +122,11 @@ 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.
@@ -134,9 +136,11 @@ 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.
+
 charset
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