Reformat node comments in plannodes.h

This is similar to d575051 but this time for the comments in
plannodes.h to avoid long lines, which is useful if adding per-field
annotations with pg_node_attr() to these planner structures.

Some patches are under discussion to add such properties to planner
fields, which is something that may or may not happen, and this change
makes future proposals easier to work on and review, which being more
consistent in style with the parse nodes.

Author: Sami Imseih
Discussion: https://postgr.es/m/Z5xTb5iBHVGns35R@paquier.xyz

Author: michaelpq