doc: make sourceRoot and setSourceRoot documentation match the im…

…plementation, fix examples

doc/languages-frameworks/rust.section.md

@@ -558,7 +558,7 @@ buildPythonPackage rec {
  hash = "sha256-miW//pnOmww2i6SOGbkrAIdc/JMDT4FJLqdMFojZeoY=";
  };
 
- sourceRoot = "source/bindings/python";
+ sourceRoot = "${src.name}/bindings/python";
 
  nativeBuildInputs = [
  cargo

doc/stdenv/stdenv.chapter.md

@@ -614,14 +614,19 @@ The list of source files or directories to be unpacked or copied. One of these m
 
 ##### `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.
 
-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"`.
+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.
+
+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}
 
 Hook executed at the start of the unpack phase.