encoding/csv: document that Writer is buffered

Add documentation that individual Write calls are buffered and
copy documentation from bufio.Writer notifying the user to call
Flush and Error when all writes are complete. Remove reference
to "file" since the implementation is general and allows any
io.Writer.

Fixes #30045

Change-Id: I50165470e548f296494e764707fbabe36c665015
Reviewed-on: https://go-review.googlesource.com/c/160680


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>