Fix SystemTap dtrace warning about smgr probe argument types.

Fix SystemTap dtrace warning about smgr probe argument types.

Commits ca326e903 and 1f8c504e3 widened the byte-count arguments of
smgr__md__read__done and smgr__md__write__done to "long long int".
SystemTap's dtrace(1) fails on that specific spelling and falls back
with a warning (misreported near the previous probe).  Use ssize_t
and size_t instead, matching the md.c call sites.

Revise probes.d's note about which types are usable as probe
arguments: recommend using system-supplied type names (macOS dtrace
rejects names like PostgreSQL's uint64), and call out "long long int"
as a known SystemTap failure case rather than a wider failure mode.

Also, update the monitoring.sgml entries for these probes,
which were missed by the prior commits.

Reported-by: Laurenz Albe <laurenz.albe@cybertec.at>
Author: Andrey Rachitskiy <pl0h0yp1@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/697d3c88568442cd637d8453d129f1bb14bbd2a8.camel@cybertec.at