Move the compiler flags in the unstable book to the rustc book

[DevQps]

Description

Currently the unstable book contains three sections:

• Unstable compiler flags
• Unstable language features
• Unstable library features.

As has been mentioned in #41142 it might be better to move the unstable compiler flags to the rustc book. I see the following reasons why:

• Compiler flags are already mentioned in the rustc book to keep documentation close together I believe it would make sense to keep all compiler flags close together. In order to avoid documentation duplication (which is hard to maintain), I believe placing them in the rustc book is more logical then not mentioning them in the rustc book and only having them in the unstable book.

• It seems that the unstable compiler flags in the unstable book are not maintained, unless there are only two unstable compiler flags (which I believe is not true since -Z lists many options).

I'd like to hear your opinion on this and, if you agree, under which section we should place this documentation. We could also aim to automize the generation of these nightly compiler flags?