pgsql: GUC: adjust effective_cache_size docs and SQL description

GUC: adjust effective_cache_size docs and SQL description

Clarify that effective_cache_size is both kernel buffers and shared
buffers.

Reported-by: nat@makarevitch.org

Discussion: /messages/by-id/153685164808.22334.15432535018443165207@wrigleys.postgresql.org

Backpatch-through: 9.3

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/75f1abfa1516e0320e56de211b778f46f3874ff5

Modified Files
--------------
doc/src/sgml/config.sgml | 3 ++-
src/backend/utils/misc/guc.c | 7 +++----
2 files changed, 5 insertions(+), 5 deletions(-)