Doc: shared_memory_size_in_huge_pages with the "SHOW" command.

Hello,

Add a reference method for shared_memory_size_in_huge_pages
with the "SHOW" command.

The current documentation explains the use of the postgres -C command,
but this method may be limited in DBaaS or managed service environments.
In particular, CloudNativePG does not allow the server to be stopped,
making the execution of $ postgres -C impossible.

Alternatively, the "SHOW" command can be used, though the documentation 
states,
"Note that the server must be shut down to view this runtime-computed 
parameter.[1]"
This wording could be misunderstood to imply that the parameter cannot 
be accessed
while the server is running.

[1] - 
https://www.postgresql.org/docs/devel/kernel-resources.html#LINUX-HUGE-PAGES

Regards,
--
Yuki Seino
NTT DATA CORPORATION