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: https://postgr.es/m/153685164808.22334.15432535018443165207@wrigleys.postgresql.org

Backpatch-through: 9.3