Document what the XQRS backsearch skip depends on #597

#596 made XQRS._backsearch skip peaks that an earlier backsearch already rejected. This is safe because of two properties of the current code:

• _update_noise doesn't recalculate qrs_thr, so a backsearch result only changes after _update_qrs.
• _update_qrs sets last_qrs_peak_num = self.peak_num (the current peak) rather than the peak_num argument.

Both look like things someone might change later, for example to match Pan-Tompkins more closely. If someone does, the skip could break without any test catching it.

The comment above the skip in _backsearch should name these dependencies, for example:

# Skip peaks a previous backsearch already rejected. A backsearch result
# depends only on qrs_thr and last_qrs_ind, which change only in
# _update_qrs, and _update_qrs moves last_qrs_peak_num to the current
# peak. If either of these stops being true, remove this skip.