Auto merge of #114969 - kpreid:dropdoc, r=Mark-Simulacrum

Go into more detail about panicking in drop.

This patch was sitting around in my drafts. I don't recall the motivation, but I think it was someone expressing confusion over “will likely abort” (since, in fact, a panicking drop _not_ caused by dropping while panicking will predictably _not_ abort).

I hope that the new text will leave people well-informed about why not to panic and when it is reasonable to panic.