doc: add performance warning to require.extensions

Discourage using require.extensions because it slows down the module
loader.  The number of file system operations that the module system
has to perform in order to resolve a `require(...)` statement to a
filename is proportional to the number of registered extensions.

PR-URL: https://github.com/nodejs/node/pull/9196


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>