syscall: document NewCallback and NewCallbackCDecl limitations

Currently NewCallback and NewCallbackCDecl may only be called a limited
number of times in a single Go process, but this property of the API is
not documented. This change fixes that, but does not document the
precise limit to avoid making that limit part of the API, leaving us
open to increasing or decreasing the limit in the future as needed.

Although the API avoids documenting a limit, it does guarantee a minimum
callback count so users can rely on at least some amount of callbacks
working.

Updates #46184.

Change-Id: I5129bf5fe301efff73ac112ba1f207ab32058833
Reviewed-on: https://go-review.googlesource.com/c/go/+/321133


Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>