kingst-la2016: update developer comments, motivated by LA5032 support

User feedback in bug #1805 provides information on the sample memory
layout for the 32-channel model in the Kingst LA series. Adjust comments
for improved readability/maintainability, and for awareness of variants
across different models.

Rework the send_chunk() comment. This routine deals with captures to the
device's internal RAM. Memory layout differs quite a lot between models
with 16 and with 32 channels.

Rephrase the stream_data() comment. This routine deals with continuous
streams of uncompressed data which bypasses the device's local memory.

Reported-By: Roman Shishkin <redacted>