doc: Fix glossary entry for data checksums workers

doc: Fix glossary entry for data checksums workers

The glossary entry for data checksums workers incorrectly stated that
they were auxiliary processes, but they are implemented as background
workers.  Fix, and while there, simplify the entry by combining the
worker and launcher into a single glossary term.  Backpatch down to
v19 where online checksums were introduced.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwEv-C9ia+rBYyePzO8F=5FVvS412ZqcOupazuOb5RafNg@mail.gmail.com
Backpatch-through: 19