math/big: update comment in the implementation of FloatPrec

math/big: update comment in the implementation of FloatPrec

Follow-up on CL 539299: missed to incorporate the updated
comment per feedback on that CL.

For #50489.

Change-Id: Ib035400038b1d11532f62055b5cdb382ab75654c
Reviewed-on: https://go-review.googlesource.com/c/go/+/542115
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>