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
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/2a279b6255c2a7a1938b85dc5c9d787b59eefee0

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