encoding/json/jsontext: rename Encoder.UnusedBuffer as Encoder.AvailableBuffer

encoding/json/jsontext: rename Encoder.UnusedBuffer as Encoder.AvailableBuffer

This follows the precedent set by:

	bufio.Writer.AvailableBuffer
	bytes.Buffer.AvailableBuffer

both with methods that return a zero-length buffer that
is intended to only be used with a following Write call.

This keeps the older UnusedBuffer method around so that
at least one commit that has both methods for migration purposes.

Updates #71497

Change-Id: I3815f593e09f645280ae5ad9cbdd63a6c147123b
Reviewed-on: https://go-review.googlesource.com/c/go/+/683896
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>