doc: add *-inl.h include rule to C++ style guide

PR-URL: https://github.com/nodejs/node/pull/16548


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>