NaN behavior

postgres=# select 'NaN'::numeric = 'NaN'::numeric,                 'NaN'::float8 = 'NaN'::float8;?column? | ?column? 
----------+----------t        | t
(1 row)

This behavior is inconsistent with most people's notion of "NaN" -- in
particular, it is inconsistent with IEEE754. I can understand why
Postgres behaves this way, and we probably can't easily change it (if we
want to continue indexing NaN values, that is), but I think it should at
least be discussed in the documentation.

Comments? I'll write up a doc patch, barring any objections.

-Neil