Fix QRegExpValidator::validate docs about the pos parameter

Fix QRegExpValidator::validate docs about the pos parameter

The code sets it to input.length() iff the regexp doesn't
match the string, while the docs say it's *always* set.

Therefore, make the docs match what the code does and
add a simple test to enforce it.

We're not changing the code to match the docs because

1) it's better to stay conservative (we don't want
to break existing behaviour);

2) this behaviour mimics what the int/double validators do
(they don't move pos at all).

Change-Id: I958074558de6b0fc5944101c6535fc7e00442ae9
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>