pgsql: Improve documentation about use of Linux huge pages.

Improve documentation about use of Linux huge pages.

Show how to get the system's huge page size, rather than misleadingly
referring to PAGE_SIZE (which is usually understood to be the regular
page size).  Show how to confirm whether huge pages have been allocated.
Minor wordsmithing.  Back-patch to 9.4 where this section appeared.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1885c88459698251eca64f095d9942c540ba0fa8

Modified Files
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doc/src/sgml/runtime.sgml | 62 +++++++++++++++++++++++++++++------------------
1 file changed, 38 insertions(+), 24 deletions(-)