document `pub(restricted)` syntax

[nikomatsakis]

Before we can stabilize pub(restricted), it needs to be documented.

cc @rust-lang/docs -- can one of you elaborate on this issue to spell out the steps needed? Even better, update the stabilization guide recently added to https://forge.rust-lang.org/ while you are at it.

[steveklabnik]

I will send in a PR to update the guide.

TL;DR:

• remove flag from the unstable book
• integrate any docs there into other docs as appropriate.

[nikomatsakis]

PRs from @cramertj:

• Add pub(restricted) docs reference#12
• Add pub(restricted) example rust-by-example#856

[steveklabnik]

I believe all of this has landed? The RBE PR can't land until the release happens, so it being open is enough. All other PRs have been merged.

[petrochenkov]

The RBE PR can't land until the release happens, so it being open is enough.

👍