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

Author: nathan-bossart

doc/src/sgml/runtime.sgml

@@ -1429,11 +1429,10 @@ export PG_OOM_ADJUST_VALUE=0
     with <varname>CONFIG_HUGETLBFS=y</varname> and
     <varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to configure
     the operating system to provide enough huge pages of the desired size.
-    To determine the number of huge pages needed, use the
-    <command>postgres</command> command to see the value of
-    <xref linkend="guc-shared-memory-size-in-huge-pages"/>.  Note that the
-    server must be shut down to view this runtime-computed parameter.
-    This might look like:
+    The runtime-computed parameter
+    <xref linkend="guc-shared-memory-size-in-huge-pages"/> reports the number
+    of huge pages required.  This parameter can be viewed before starting the
+    server with a <command>postgres</command> command like:
 <programlisting>
 $ <userinput>postgres -D $PGDATA -C shared_memory_size_in_huge_pages</userinput>
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