doc: remove example labels from buffer.md

Remove instances of `Example:` that introduce code that is
self-evidently example code. Move descriptive text about examples into
comments in the code. Wrap adjacent text to 80 characters.

PR-URL: https://github.com/nodejs/node/pull/19582


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>