file_server: clarify precompressed (#224)

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

@@ -30,7 +30,9 @@ file_server [<matcher>] [browse] {
 - **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.
-- **precompressed** is the list of encoding formats to search for precompressed sidecar files. Arguments are an ordered list of encoding formats to search for precompressed sidecar files. Supported formats are `gzip`, `zstd` and `br`.
+- **precompressed** is the list of encoding formats to search for precompressed sidecar files. Arguments are an ordered list of encoding formats to search for precompressed [sidecar files](https://en.wikipedia.org/wiki/Sidecar_file). Supported formats are `gzip` (`.gz`), `zstd` (`.zst`) and `br` (`.br`).
+
+  All file lookups will look for the existence of the uncompressed file first. Once found Caddy will look for sidecar files with the file extension of each enabled format. If a precompressed sidecar file is found, Caddy will respond with the precompressed file, with the `Content-Encoding` response header set appropriately. Otherwise, Caddy will respond with the uncompressed file as normal. If the [`encode` directive](/docs/caddyfile/directives/encode) is enabled, then it may compress the response on-the-fly if not precompressed.
 - **status** is an optional status code override to be used when writing the response. Particularly useful when responding to a request with a custom error page. Can be a 3-digit status code, For example: `404`. Placeholders are supported. By default, the written status code will typically be `200`, or `206` for partial content.
 - **disable_canonical_uris** disables the default behaviour of redirecting to add a trailing slash if the request path is a directory, or remove the trailing slash if the request path is a file. Note that by default, canonicalization will not happen if the last element of the request's path (the filename) underwent an internal rewrite, to avoid clobbering an explicit rewrite with implicit behaviour.
 - **pass_thru** enables pass-thru mode, which continues to the next HTTP handler in the route if the requested file is not found, instead of returning a `404`. Practically, this is likely only be useful inside of a [`route`](/docs/caddyfile/directives/route) block, because the `file_server` directive is effectively [ordered last](/docs/caddyfile/directives#directive-order) otherwise.