Try to clarify how crates are able to access force-unstable dependencies

without a feature attribute.

src/stability.md

@@ -99,12 +99,12 @@ default `allow`, but most of the standard library raises it to a warning with
 ## -Zforce-unstable-if-unmarked
 
 The `-Zforce-unstable-if-unmarked` flag has a variety of purposes to help
-enforce that the correct crates are marked as unstable, but can still use
-private crates without special attributes. It was introduced primarily to
-allow rustc and the standard library to link to arbitrary crates on crates.io
-which do not themselves use `staged_api`. `rustc` also relies on this flag to
-mark all of its crates as unstable with the `rustc_private` feature so that
-each crate does not need to be carefully marked with `unstable`.
+enforce that the correct crates are marked as unstable. It was introduced
+primarily to allow rustc and the standard library to link to arbitrary crates
+on crates.io which do not themselves use `staged_api`. `rustc` also relies on
+this flag to mark all of its crates as unstable with the `rustc_private`
+feature so that each crate does not need to be carefully marked with
+`unstable`.
 
 This flag is automatically applied to all of `rustc` and the standard library
 by the bootstrap scripts. This is needed because the compiler and all of its
@@ -115,7 +115,11 @@ This flag has the following effects:
 - Marks the crate as "unstable" with the `rustc_private` feature if it is not
   itself marked as stable or unstable.
 - Allows these crates to access other forced-unstable crates without any need
-  for attributes.
+  for attributes. Normally a crate would need a `#![feature(rustc_private)]`
+  attribute to use other unstable crates. However, that would make it
+  impossible for a crate from crates.io to access its own dependencies since
+  that crate won't have a `feature(rustc_private)` attribute, but *everything*
+  is compiled with `-Zforce-unstable-if-unmarked`.
 
 Code which does not use `-Zforce-unstable-if-unmarked` should include the
 `#![feature(rustc_private)]` crate attribute to access these force-unstable