doc: correct buffer changelog ordering

This commit reorders the function level changelogs of
Buffer.prototype.fill() and Buffer.alloc() to reflect the order
in which the entries were added.

PR-URL: https://github.com/nodejs/node/pull/18129
Fixes: https://github.com/nodejs/node/issues/18128


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>