DOC: quic: fix default minimal value for max window size

DOC: quic: fix default minimal value for max window size

It is possible to override the default QUIC congestion algorithm on a
bind line. With the same setting, it is also possible to specify the
maximum congestion window size.

The parser rejects values outside of the range between 10k and 4g. This
is in contradiction with the documentation which specify 1k as the lower
value. Correct this value in the documentation.

This should be backported up to 2.9.