pgsql: Improve comments about semijoin implementation strategy, per a

Log Message:
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Improve comments about semijoin implementation strategy, per a question
from Robert Haas.

Modified Files:
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pgsql/src/backend/optimizer/path:
joinrels.c (r1.97 -> r1.98)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/joinrels.c?r1=1.97&r2=1.98)