pgsql: Revise the header comment for match_clause_to_indexcol()

Revise the header comment for match_clause_to_indexcol()

Since d4378c0005e6, match_clause_to_indexcol() doesn't always return NULL
for an OR clause.  This commit reflects that in the function header comment.

Reported-by: Pavel Borisov <pashkin.elfe@gmail.com>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/23ef119f58e3eaa29bedc69320fd61b108ab8c10

Modified Files
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src/backend/optimizer/path/indxpath.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)