reduce overlong lines

Author: tshepang

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

@@ -19,11 +19,16 @@ There are three types of tools you can write in bootstrap:
 
   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.
+  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.
+Regardless of the tool type,
+you must return `ToolBuildResult` from the tool’s [`Step`] implementation,
+and use `ToolBuild` inside it.
 
 [`Step`]: https://doc.rust-lang.org/nightly/nightly-rustc/bootstrap/core/builder/trait.Step.html