Improve "visibility" of rustdoc books ("stable" and unstable ones)

[GuillaumeGomez]

Currently, very few people know the existence of https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/ and of https://doc.rust-lang.org/rustdoc/what-is-rustdoc.html, which is a shame considering how huge is the amount of available options and tips.

I suggested adding those links (the unstable one only in unstable build of course) at the beginning of the "help display".

[AndyGauge]

I'm going to try to implement this.

[AndyGauge]

andy@DESKTOP-T75H07P:~/rust/build/x86_64-unknown-linux-gnu/stage2/bin$ ./rustdoc --help
See http://doc.rust-lang.org/rustdoc for more information
rustdoc [options] <input>

Options:

I was able to get rustdoc to compile with a little tooltip!