pgsql: Add some notes about the basic mathematical laws that the system

Log Message:
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Add some notes about the basic mathematical laws that the system presumes
hold true for operators in a btree operator family.  This is mostly to
clarify my own thinking about what the planner can assume for optimization
purposes.  (blowing dust off an old abstract-algebra textbook...)

Modified Files:
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    pgsql/src/backend/access/nbtree:
        README (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/README.diff?r1=1.16&r2=1.17)