doc: clarify automatic encoding detection

Fixes #1103

CHANGELOG.md

@@ -27,6 +27,8 @@ Bug fixes:
   `**` is now accepted as valid syntax anywhere in a glob.
 * [BUG #1095](https://github.com/BurntSushi/ripgrep/issues/1095):
   Fix corner cases involving the `--crlf` flag.
+* [BUG #1103](https://github.com/BurntSushi/ripgrep/issues/1103):
+  Clarify what `--encoding auto` does.
 * [BUG #1106](https://github.com/BurntSushi/ripgrep/issues/1106):
   `--files-with-matches` and `--files-without-match` work with one file.
 * [BUG #1093](https://github.com/BurntSushi/ripgrep/pull/1093):

GUIDE.md

@@ -609,7 +609,8 @@ topic, but we can try to summarize its relevancy to ripgrep:
   the most popular encodings likely consist of ASCII, latin1 or UTF-8. As
   a special exception, UTF-16 is prevalent in Windows environments
 
-In light of the above, here is how ripgrep behaves:
+In light of the above, here is how ripgrep behaves when `--encoding auto` is
+given, which is the default:
 
 * All input is assumed to be ASCII compatible (which means every byte that
   corresponds to an ASCII codepoint actually is an ASCII codepoint). This

src/app.rs

@@ -982,10 +982,15 @@ fn flag_encoding(args: &mut Vec<RGArg>) {
     const LONG: &str = long!("\
 Specify the text encoding that ripgrep will use on all files searched. The
 default value is 'auto', which will cause ripgrep to do a best effort automatic
-detection of encoding on a per-file basis. Other supported values can be found
-in the list of labels here:
+detection of encoding on a per-file basis. Automatic detection in this case
+only applies to files that begin with a UTF-8 or UTF-16 byte-order mark (BOM).
+No other automatic detection is performend.
+
+Other supported values can be found in the list of labels here:
 https://encoding.spec.whatwg.org/#concept-encoding-get
 
+For more details on encoding and how ripgrep deals with it, see GUIDE.md.
+
 This flag can be disabled with --no-encoding.
 ");
     let arg = RGArg::flag("encoding", "ENCODING").short("E")