pgsql: Doc: update pg_list.h header comments to include XidLists.

Doc: update pg_list.h header comments to include XidLists.

I realize that the XidList infrastructure is rather incomplete,
but failing to mention it in adjacent comments takes that a bit
too far.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0efecb5518c914a45d9f294adebc2948f5882f2e

Modified Files
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src/include/nodes/pg_list.h | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)