Document the file-modification-indicator statusline element (#6036)

book/src/configuration.md

@@ -98,6 +98,7 @@ The following statusline elements can be configured:
 | `spinner` | A progress spinner indicating LSP activity |
 | `file-name` | The path/name of the opened file |
 | `file-base-name` | The basename of the opened file |
+| `file-modification-indicator` | The indicator to show whether the file is modified (a `[+]` appears when there are unsaved changes) |
 | `file-encoding` | The encoding of the opened file if it differs from UTF-8 |
 | `file-line-ending` | The file line endings (CRLF or LF) |
 | `total-line-numbers` | The total line numbers of the opened file |