strfmon(3): Fix # explanation

There's only one value that specifies the number of digits after the
decimal point (oh, sorry, the "radix character") the other specifies the
number before...

While here, add a little more info on the effects of using the #n value.

Obtained from: https://github.com/NetBSD/src/commit/d1dd1a086400ae719bde1f2c45938d9bc1d29e8b
Reviewed by: kib
PR: 267282
Github PR: #619
MFC after: 1 week