pow.3: Minor tweak to BUGS

Remove the text ("rare)" after a note from Vincent Lefèvre:

Subject: [Bug math/13932] dbl-64 pow unexpectedly slow for some inputs
Date: Sat, 23 May 2020 21:31:52 +0000
From: vincent-srcware at vinc17 dot net <sourceware-bugzilla@sourceware.org>
To: mtk.manpages@gmail.com

https://sourceware.org/bugzilla/show_bug.cgi?id=13932

--- Comment #26 from Vincent Lefèvre <vincent-srcware at vinc17 dot net> ---
(In reply to Michael Kerrisk from comment #25)
> Fix documented for man-pages-5.07.
[...]
> -On 64-bits,
> +Before glibc 2.28,
>  .\"
>  .\" https://sourceware.org/bugzilla/show_bug.cgi?id=13932
> +on some architectures (e.g., x86-64)
>  .BR pow ()
>  may be more than 10,000 times slower for some (rare) inputs
>  than for other nearby inputs.
[...]

The problematic values are uncommon, but not so rare, in the sense
that they are close to simple values, i.e. are likely to occur in
practice. An example given above: pow(0.999999999999999889, 1.5)

1 and 1.5 are very simple values, which are more likely to occur
in practice than some fixed random value. Then it suffices to have
a small rounding error on 1...

For instance, this is very different from hard-to-round cases of
exp, which are also very slow IMHO, but unless one writes a
specific program for them, no-one should notice the slowness
because such a case would typically occur only once among billions
(I don't remember the accuracy before the slowest path in this
library).

Reported-by: Vincent Lefèvre <vincent-srcware@vinc17.net>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/pow.3

@@ -334,7 +334,7 @@ Before glibc 2.28,
 .\" https://sourceware.org/bugzilla/show_bug.cgi?id=13932
 on some architectures (e.g., x86-64)
 .BR pow ()
-may be more than 10,000 times slower for some (rare) inputs
+may be more than 10,000 times slower for some inputs
 than for other nearby inputs.
 This affects only
 .BR pow (),