Improve docs for file_server and add caddy.fs namespace

src/docs/markdown/caddyfile/directives/file_server.md

@@ -8,7 +8,7 @@ A static file server. It works by appending the request's URI path to the [site'
 
 By default, it enforces canonical URIs; meaning HTTP redirects will be issued for requests to directories that do not end with a trailing slash (to add it) or requests to files that have a trailing slash (to remove it). Redirects are not issued, however, if an internal rewrite modifies the last element of the path (the filename).
 
-Most often, the `file_server` directive is paired with the [`root`](/docs/caddyfile/directives/root) directive to set file root for the whole site.
+Most often, the `file_server` directive is paired with the [`root`](/docs/caddyfile/directives/root) directive to set file root for the whole site. A site root does not carry sandbox guarantees: the file server does prevent directory traversal, but symbolic links within the root can still allow accesses outside of the root.
 
 ## Syntax
 
@@ -26,7 +26,7 @@ file_server [<matcher>] [browse] {
 ```
 
 - **browse** enables file listings for requests to directories that do not have an index file.
-- **root** sets the path to the site root for just this file server instance, overriding any other. Default: `{http.vars.root}` or the current working directory. Note: This subdirective only changes the root for this directive. For other directives (like [`try_files`](/docs/caddyfile/directives/try_files) or [`templates`](/docs/caddyfile/directives/templates)) to know the same site root, use the [`root`](/docs/caddyfile/directives/root) directive, not this subdirective.
+- **root** sets the path to the site root. It's similar to the [`root`](/docs/caddyfile/directives/root) directive except it applies to this file server instance only and overrides any other site root that may have been defined. Default: `{http.vars.root}` or the current working directory. Note: This subdirective only changes the root for this directive. For other directives (like [`try_files`](/docs/caddyfile/directives/try_files) or [`templates`](/docs/caddyfile/directives/templates)) to know the same site root, use the [`root`](/docs/caddyfile/directives/root) directive, not this subdirective.
 - **hide** is a list of files or folders to hide; if requested, the file server will pretend they do not exist. Accepts placeholders and glob patterns. Note that these are _file system_ paths, NOT request paths. In other words, relative paths use the current working directory as a base, NOT the site root; and all paths are transformed to their absolute form before comparisons (if possible). Specifying a file name or pattern without a path separator will hide all files with a matching name regardless of its location; otherwise, a path prefix match will be attempted, and then a globular match. Since this is a Caddyfile config, the active configuration file(s) will be added by default.
 - **index** is a list of filenames to look for as index files. Default: `index.html index.txt`
 - **<template_file>** is an optional custom template file to use for directory listings. Defaults to the template that can be found [here in the source code ![external link](/resources/images/external-link.svg)](https://github.com/caddyserver/caddy/blob/master/modules/caddyhttp/fileserver/browse.html). Browse templates can use actions from [the standard templates module](/docs/modules/http.handlers.templates#docs) as well.

src/docs/markdown/extending-caddy/namespaces.md

@@ -15,6 +15,7 @@ Documentation for non-standard module namespaces can be found with the documenta
 Namespace | Expected Type | Description
 --------- | ------------- | -----------
 |         | [`caddy.App`](https://pkg.go.dev/github.com/caddyserver/caddy/v2?tab=doc#App) | Caddy app
+caddy.fs | [`fs.StatFS`](https://pkg.go.dev/io/fs#StatFS) | Virtual file system
 caddy.logging.encoders.filter | [`logging.LogFieldFilter`](https://pkg.go.dev/github.com/caddyserver/caddy/v2/modules/logging?tab=doc#LogFieldFilter) | Log field filter</i>
 caddy.logging.writers | [`caddy.WriterOpener`](https://pkg.go.dev/github.com/caddyserver/caddy/v2?tab=doc#WriterOpener) | Log writers
 caddy.storage | [`caddy.StorageConverter`](https://pkg.go.dev/github.com/caddyserver/caddy/v2?tab=doc#StorageConverter) | Storage backends