[docs] Import warnings that currently live in gists.

[zpao]

The goal is to make it so we're not locked in to these existing as a gist on on any single person's account. This way if we want to make changes to the contents while 1 person is unavailable, it's totally doable. There are other ways we could solve that (easiest if we wanted to keep this on github is a standalone repo with each warning getting a file)

Downside: There is currently some discussion happening on the gists, which could be helpful and it's a slightly longer turnaround to updating the content. Not sure what we should do about that, if anything.

I grepped for all the unique fb.me links in master (15-stable matches) and opened all of them. These 4 were the ones that don't already link to some part of our docs (1 currently links to a specific issue which is probably ok). Only some minor changes were made to keep formatting and markdown consistency. We should probably do a pass over previous version (0.14, 0.13 should be enough) and do the same. I can do that if we want to take this.

[gaearon]

Downside: There is currently some discussion happening on the gists, which could be helpful

Tbh I think it’s confusing because we don’t get notified about it and mentions don’t work either. Let’s keep it here 👍

[zpao]

Taking @gaearon's comment as an accept. I updated with the latest content from the unknown-prop gist and went back through 0.14 to ensure we didn't have anything there. I'll get this shipped and update the fb.me links to redirect appropriately.