doc: Improve postgres command for shared_memory_size_in_huge_pages

doc: Improve postgres command for shared_memory_size_in_huge_pages

The command used in the documentation to retrieve the value of the
runtime-computed GUC shared_memory_size_in_huge_pages would also show to
the user all the log messages generated by the postmaster before and
after printing the wanted value.  This can be confusing, as the wanted
result could be masked with a lot of noise.

One way to avoid those log messages is to use something like "-c
log_min_messages=fatal" in the command (my idea, but that's not common
knowledge).  Rather than mentioning this option, suffix the command with
a redirection of stderr to /dev/null, which is the stream location where
the logs show up.  This is enough to show only the GUC value to the
user when copy-pasting the command.

Reported-by: Magnus Hagander
Author: Nathan Bossart
Discussion: https://postgr.es/m/20220314173417.GA1020555@nathanxps13