ci: Document what makes certain tasks special

ci: Document what makes certain tasks special

To increase coverage without drastically increasing CI resource usage, we have
different CI tasks test different things (e.g. the linux tasks use
sanitizers).  Unfortunately that can create confusing situations where CI
fails on some OS, but not others, without the problem appearing to be platform
dependent.

To, partially, address that, add a comment, prefixed with SPECIAL, to each
task that we use to test in some non-default way.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/321570.1741195755@sss.pgh.pa.us