Update Length doc

Author: VincentLanglet

reference/constraints/Length.rst

@@ -201,6 +201,8 @@ You can use the following parameters in this message:
 Parameter               Description
 ======================  ============================================================
 ``{{ limit }}``         The expected maximum length
+``{{ min }}``           The expected minimum length
+``{{ max }}``           The expected maximum length
 ``{{ value }}``         The current (invalid) value
 ``{{ value_length }}``  The current value's length
 ======================  ============================================================
@@ -233,6 +235,8 @@ You can use the following parameters in this message:
 Parameter               Description
 ======================  ============================================================
 ``{{ limit }}``         The expected minimum length
+``{{ min }}``           The expected minimum length
+``{{ max }}``           The expected maximum length
 ``{{ value }}``         The current (invalid) value
 ``{{ value_length }}``  The current value's length
 ======================  ============================================================