Adjust documentation for configuring Linux huge pages.

Adjust documentation for configuring Linux huge pages.

The present wording about viewing shared_memory_size_in_huge_pages
seems to suggest that the parameter cannot be viewed after startup
at all, whereas the intent is to make it clear that you can't use
"postgres -C" to view this parameter while the server is running.
This commit rephrases this section to remove the ambiguity.

Author: Seino Yuki
Reviewed-by: Michael Paquier, David G. Johnston, Fujii Masao
Discussion: https://postgr.es/m/420584fd274f9ec4f337da55ffb3b790%40oss.nttdata.com
Backpatch-through: 15