bufio: clarify SplitFunc docs for nil token
bufio: clarify SplitFunc docs for nil token

Fixes #25472

Change-Id: Idb72ed06a3dc43c49ab984a80f8885352b036465
Reviewed-on: https://go-review.googlesource.com/118695
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

CAFxX/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,josharian/go,nawawi/go,nawawi/go,cloudflare/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go,josharian/go,josharian/go,cloudflare/go,akutz/go,josharian/go,nawawi/go,nawawi/go,akutz/go,golang/go,josharian/go,akutz/go,nawawi/go,golang/go,nawawi/go,golang/go,CAFxX/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,golang/go,akutz/go,akutz/go,golang/go,CAFxX/go,josharian/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go