doc: add missing types in buffer.md

PR-URL: https://github.com/nodejs/node/pull/27030


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>