Incorrect comment on NewBufferedWriter?

The comment (and docs) to `NewBufferedWriter` state that "Users must call Close to guarantee all data has been forwarded to the underlying io.Writer": https://github.com/golang/snappy/blob/674baa8c7fc30da5df3074a459494a7e6b427dff/encode.go#L132

However, in looking at the Flush/Close implementations, and their usages in [tests](https://github.com/golang/snappy/blob/3ff355f7bbcd12d666f5a3ad9aaa9ae5aff7a069/snappy_test.go#L766), it seems that users only need to call `Flush`, not `Close`, which would allow a caller to continue using the buffered writer.

Start with the NewBufferedWriter comment in encode.go and compare its wording with the Flush and Close implementations. Review the related usage in snappy_test.go, then update the comment and associated documentation to match the supported usage. Confirm the existing buffered-writer tests still pass.

Written by the indexing model from the issue text.