strconv: document parsing of leading +/-

Explicitly document the handling of a sign prefix, and the interaction
between the sign and base prefixes.

Fixes #46641.

Change-Id: I3cd6773e3f074fe671a944a05a79d2408137fcd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/325875


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>