Fix the wording in "strconv" and "fmt" to make explicit
that the "g" and "G" formats remove trailing zeroes.

Fixes #25082

Change-Id: I2e2ad0a98d2ea27a3a8a006a0563b366f7a3b71b
Reviewed-on: https://go-review.googlesource.com/127135Reviewed-by: Rob Pike <r@golang.org>