Document virality of feature(rustc_private)

Since 9cb1998 this feature is viral.

src/doc/unstable-book/src/language-features/rustc-private.md

@@ -6,6 +6,9 @@ The tracking issue for this feature is: [#27812]
 
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-This feature allows access to unstable internal compiler crates.
+This feature allows access to unstable internal compiler crates such as `rustc_driver`.
 
-Additionally it changes the linking behavior of crates which have this feature enabled. It will prevent linking to a dylib if there's a static variant of it already statically linked into another dylib dependency. This is required to successfully link to `rustc_driver`.
+The presence of this feature changes the way the linkage format for dylibs is calculated in a way
+that is necessary for linking against dylibs that statically link `std` (such as `rustc_driver`).
+This makes this feature "viral" in linkage; its use in a given crate makes its use required in
+dependent crates which link to it (including integration tests, which are built as separate crates).