doc: make `sourceRoot` and `setSourceRoot` documentation match the implementation, fix examples

doc/languages-frameworks/rust.section.md

···
     hash = "sha256-miW//pnOmww2i6SOGbkrAIdc/JMDT4FJLqdMFojZeoY=";
   };
 
-  sourceRoot = "source/bindings/python";
+  sourceRoot = "${src.name}/bindings/python";
 
   nativeBuildInputs = [
     cargo

doc/stdenv/stdenv.chapter.md

···
 
 ##### `sourceRoot` {#var-stdenv-sourceRoot}
 
-After running `unpackPhase`, the generic builder changes the current directory to the directory created by unpacking the sources. If there are multiple source directories, you should set `sourceRoot` to the name of the intended directory. Set `sourceRoot = ".";` if you use `srcs` and control the unpack phase yourself.
+After unpacking all of `src` and `srcs`, if neither of `sourceRoot` and `setSourceRoot` are set, `unpackPhase` of the generic builder checks that the unpacking produced a single directory and moves the current working directory into it.
+
+If `unpackPhase` produces multiple source directories, you should set `sourceRoot` to the name of the intended directory.
+You can also set `sourceRoot = ".";` if you want to control it yourself in a later phase.
 
-By default the `sourceRoot` is set to `"source"`. If you want to point to a sub-directory inside your project, you therefore need to set `sourceRoot = "source/my-sub-directory"`.
+For example, if your want your build to start in a sub-directory inside your sources, and you are using `fetchzip`-derived `src` (like `fetchFromGitHub` or similar), you need to set `sourceRoot = "${src.name}/my-sub-directory"`.
 
 ##### `setSourceRoot` {#var-stdenv-setSourceRoot}
 
 Alternatively to setting `sourceRoot`, you can set `setSourceRoot` to a shell command to be evaluated by the unpack phase after the sources have been unpacked. This command must set `sourceRoot`.
+
+For example, if you are using `fetchurl` on an archive file that gets unpacked into a single directory the name of which changes between package versions, and you want your build to start in its sub-directory, you need to set `setSourceRoot = "sourceRoot=$(echo */my-sub-directory)";`, or in the case of multiple sources, you could use something more specific, like `setSourceRoot = "sourceRoot=$(echo ${pname}-*/my-sub-directory)";`.
 
 ##### `preUnpack` {#var-stdenv-preUnpack}