pgsql: Update comments on CustomPath struct.

Update comments on CustomPath struct.

Commit e7cb7ee14 allowed custom scan providers to create CustomPath
paths for join relations as well, but missed updating the comments.

Back-patch to all supported branches.

Discussion: https://postgr.es/m/CAPmGK15ODkN%2B%3DhkBCufj1HBW0x5OTb65Xuy7ryXchMdiCMpx_g%40mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c575e00230ed1262edb7845e2e9a22ec164bf9f0

Modified Files
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src/include/nodes/pathnodes.h | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)