pgsql: Reformat some node comments

Reformat some node comments

Use per-field comments for IndexInfo, instead of one big header
comment listing all the fields.  This makes the relevant comments
easier to find, and it will also make it less likely that comments are
not updated when fields are added or removed, as has happened in the
past.

Author: Japin Li <japinli@hotmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/ME0P300MB04453E6C7EA635F0ECF41BFCB6832%40ME0P300MB0445.AUSP300.PROD.OUTLOOK.COM

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/de5aa15209397712dcfdeee2a72977b83374f276

Modified Files
--------------
src/include/nodes/execnodes.h | 69 ++++++++++++++++++++++++-------------------
1 file changed, 38 insertions(+), 31 deletions(-)