BUG/MINOR: sample: Fix bytes() when length it greater than remaining data

BUG/MINOR: sample: Fix bytes() when length it greater than remaining data

For bytes() converter, when the length parameter is greater than the
remaining data, no truncation on length must be performed. However, in that
case, nothing was performed at all. The changes because of the offset
parameter was just ignored.

Now, when the length value is too large, the sample data are moved
accordingly to the offset value as expected.

The coresponding reg-test was updated to test this case with a non-zero
offset. In addition the documentation was fixed to properly match what the
converter do. It was wrongly updated when the support of variables was
introduced in 2.9.

This patch must be backported as far as 3.0.