docs: describe tab-width and unit subkeys (helix-editor#5862)

book/src/languages.md

@@ -56,7 +56,7 @@ These configuration keys are available:
 | `auto-format`         | Whether to autoformat this language when saving               |
 | `diagnostic-severity` | Minimal severity of diagnostic for it to be displayed. (Allowed values: `Error`, `Warning`, `Info`, `Hint`) |
 | `comment-token`       | The token to use as a comment-token                           |
-| `indent`              | The indent to use. Has sub keys `tab-width` and `unit`        |
+| `indent`              | The indent to use. Has sub keys `unit` (the text inserted into the document when indenting; usually set to N spaces or `"\t"` for tabs) and `tab-width` (the number of spaces rendered for a tab) |
 | `language-server`     | The Language Server to run. See the Language Server configuration section below. |
 | `config`              | Language Server configuration                                 |
 | `grammar`             | The tree-sitter grammar to use (defaults to the value of `name`) |