manual: Clarify return value of cbrt(3)

Link: <https://lore.kernel.org/linux-man/ZeYKUOKYS7G90SaV@debian/T/#mff0ab388000c6afdb5e5162804d4a0073de481de>
Reported-by: Morten Welinder <mwelinder@gmail.com>
Cowritten-by: Morten Welinder <mwelinder@gmail.com>
Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: Vincent Lefevre <vincent@vinc17.net>
Cc: DJ Delorie <dj@redhat.com>
Cc: Paul Zimmermann <Paul.Zimmermann@inria.fr>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: DJ Delorie <dj@redhat.com>

manual/math.texi

@@ -700,8 +700,12 @@ Mathematically, it should return a complex number.
 @standardsx{cbrtfN, TS 18661-3:2015, math.h}
 @standardsx{cbrtfNx, TS 18661-3:2015, math.h}
 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
-These functions return the cube root of @var{x}.  They cannot
-fail; every representable real value has a representable real cube root.
+These functions return the cube root of @var{x}.
+They cannot fail;
+every representable real value
+has a real cube root,
+and rounding it to a representable value
+never causes overflow nor underflow.
 @end deftypefun
 
 @deftypefun double hypot (double @var{x}, double @var{y})