strconv: document handling of NaN and ±Inf

In addition to the example that was added in 203b80ab, mention these
special cases in the doc comment. This change also adjusts the example
to include "+Inf", as it was not otherwise mentioned that the plus
symbol may be present.

Fix #30990

Change-Id: I97d66f4aff6a17a6ccc0ee2e7f32e39ae91ae454
Reviewed-on: https://go-review.googlesource.com/c/go/+/179738


Reviewed-by: Alex Miasoedov <msoedov@gmail.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>