BUG-4268: clarify length constraint API contract

The contract did not specify whether the returned constraints are
declared or effective. The intent is to return effective constraints,
which include any constraints imposed by base types -- as is the case
with range constraints.

Change-Id: I0d0582edf87c0a78db3e8664655f139f827217a7
Signed-off-by: Robert Varga <robert.varga@pantheon.sk>