lib.fileset: Move introduction section above the functions

Previously the introductory section and the function listings were in
different places. But now nixdoc supports having them together
with nix-community/nixdoc#70!

lib/fileset/default.nix

@@ -1,3 +1,52 @@
+/*
+  <!-- This anchor is here for backwards compatibity -->
+  []{#sec-fileset}
+
+  The [`lib.fileset`](#sec-functions-library-fileset) library allows you to work with _file sets_.
+  A file set is a mathematical set of local files that can be added to the Nix store for use in Nix derivations.
+  File sets are easy and safe to use, providing obvious and composable semantics with good error messages to prevent mistakes.
+
+  See the [function reference](#sec-functions-library-fileset) for function-specific documentation.
+
+  ## Implicit coercion from paths to file sets {#sec-fileset-path-coercion}
+
+  All functions accepting file sets as arguments can also accept [paths](https://nixos.org/manual/nix/stable/language/values.html#type-path) as arguments.
+  Such path arguments are implicitly coerced to file sets containing all files under that path:
+  - A path to a file turns into a file set containing that single file.
+  - A path to a directory turns into a file set containing all files _recursively_ in that directory.
+
+  If the path points to a non-existent location, an error is thrown.
+
+  ::: {.note}
+  Just like in Git, file sets cannot represent empty directories.
+  Because of this, a path to a directory that contains no files (recursively) will turn into a file set containing no files.
+  :::
+
+  :::{.note}
+  File set coercion does _not_ add any of the files under the coerced paths to the store.
+  Only the [`toSource`](#function-library-lib.fileset.toSource) function adds files to the Nix store, and only those files contained in the `fileset` argument.
+  This is in contrast to using [paths in string interpolation](https://nixos.org/manual/nix/stable/language/values.html#type-path), which does add the entire referenced path to the store.
+  :::
+
+  ### Example {#sec-fileset-path-coercion-example}
+
+  Assume we are in a local directory with a file hierarchy like this:
+  ```
+  ├─ a/
+  │  ├─ x (file)
+  │  └─ b/
+  │     └─ y (file)
+  └─ c/
+     └─ d/
+  ```
+
+  Here's a listing of which files get included when different path expressions get coerced to file sets:
+  - `./.` as a file set contains both `a/x` and `a/b/y` (`c/` does not contain any files and is therefore omitted).
+  - `./a` as a file set contains both `a/x` and `a/b/y`.
+  - `./a/x` as a file set contains only `a/x`.
+  - `./a/b` as a file set contains only `a/b/y`.
+  - `./c` as a file set is empty, since neither `c` nor `c/d` contain any files.
+*/
 { lib }:
 let