pgsql: Fix obsolete _bt_first comments.

Fix obsolete _bt_first comments.

_bt_first doesn't necessarily hold onto a buffer pin on success exit.
Fix header comments that claimed that we'll always hold onto a pin.

Oversight in commit 2ed5b87f96.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/846cfe0dcc0281fd9064798ac3539e20379b9b91

Modified Files
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src/backend/access/nbtree/nbtsearch.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)