doc: improve prepositions in buffer.md

* `at which to begin copying to` -> `at which to begin writing`
* `at which to begin copying from` -> `from which to begin copying`
* wrap at 80 chars

PR-URL: https://github.com/nodejs/node/pull/19817


Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>