Rollup merge of #146127 - Kobzol:rename-tool-rustc, r=jieyouxu

Rename `ToolRustc` to `ToolRustcPrivate`

I think that this name gets the point across much better.

r? ````@jieyouxu````

Author: Zalathar

src/building/bootstrapping/writing-tools-in-bootstrap.md

@@ -11,11 +11,8 @@ There are three types of tools you can write in bootstrap:
   Use this for tools that rely on the locally built std. The output goes into the "stageN-tools" directory.
   This mode is rarely used, mainly for `compiletest` which requires `libtest`.
 
-- **`Mode::ToolRustc`**
-  Use this for tools that depend on both the locally built `rustc` and the target `std`. This is more complex than
-  the other modes because the tool must be built with the same compiler used for `rustc` and placed in the "stageN-tools"
-  directory. When you choose `Mode::ToolRustc`, `ToolBuild` implementation takes care of this automatically.
-  If you need to use the builder’s compiler for something specific, you can get it from `ToolBuildResult`, which is
+- **`Mode::ToolRustcPrivate`**
+  Use this for tools that use the `rustc_private` mechanism, and thus depend on the locally built `rustc` and its rlib artifacts. This is more complex than the other modes because the tool must be built with the same compiler used for `rustc` and placed in the "stageN-tools" directory. When you choose `Mode::ToolRustcPrivate`, `ToolBuild` implementation takes care of this automatically. If you need to use the builder’s compiler for something specific, you can get it from `ToolBuildResult`, which is
   returned by the tool's [`Step`].
 
 Regardless of the tool type you must return `ToolBuildResult` from the tool’s [`Step`] implementation and use `ToolBuild` inside it.