src: document the Node.js context emebdder data
Just noticed that they are never properly documented and I found myself explaining to others from the top of my head again. So might as well just add some comments to document them.
CI: https://ci.nodejs.org/job/node-test-pull-request/59991/
Commit message typo: emebdder -> embedder
CI: https://ci.nodejs.org/job/node-test-pull-request/60100/
@VoltrexKeyva @addaleax @legendecas PTAL again, thanks!
CI: https://ci.nodejs.org/job/node-test-pull-request/60103/
Landed in 8825dc321afcee9dc0cc209e5bd54b0397848914