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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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b0051f1d4c9f6db51b4a8b7ba8978548cfbc0918

Modified Files
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doc/src/sgml/runtime.sgml | 63 +++++++++++++++++++++++++++++++----------------
1 file changed, 42 insertions(+), 21 deletions(-)