Re: Ryu floating point output patch

Latest patch.

No semantic changes; fixes postgresql.conf.sample and adds docs.

In the absence of feedback to the contrary, I've documented the current
behavior of extra_float_digits. I've also removed language from the docs
regarding non-IEEE float formats, and made it clear that all supported
platforms now use IEEE format (whether or not they implement the actual
arithmetic correctly).

I thought of what might be a good reason to leave float->numeric
conversions alone: since they're currently immutable (unlike float
output), they might appear in indexes, and pg_upgrade would have to cope
with that. I'm therefore going to consider float to numeric conversion
as being an issue for a separate patch if at all, rather than part of
this one.

-- 
Andrew (irc:RhodiumToad)