pgsql: Fix outdated comments, GIST search queue is not an RBTree anymor

Fix outdated comments, GIST search queue is not an RBTree anymore.

The GiST search queue is implemented as a pairing heap rather than as
Red-Black Tree, since 9.5 (commit e7032610). I neglected these comments
in that commit.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/45310221a9afccd98e78813459472370ade9dc4c

Modified Files
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src/backend/access/gist/gistscan.c |  4 ++--
src/include/access/gist_private.h  | 18 +++++++-----------
2 files changed, 9 insertions(+), 13 deletions(-)