Add CSS unit reference for text style.

docs/styles/css_units/text_style.md

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+# Text style
+
+The text style unit is a combination of any of the legal text style values in a space-separated list.
+
+!!! warning
+
+    Not to be confused with the [`text-style`](../text_style.md) CSS rule that sets the style of text in a widget.
+
+## Syntax
+
+--8<-- "docs/styles/snippets/text_style_syntax.md"
+
+## Examples
+
+!!! note
+
+    The `text-style` CSS rule and the text style unit are closely related but they are not the same thing.
+    Here, we show examples of the text style unit, which are relevant for [all CSS rules](#used-by) that use the text style unit.
+
+```css
+Widget {
+    text-style: bold;
+    text-style: italic;
+    text-style: reverse;
+    text-style: underline;
+    text-style: strike;
+
+    /* When the unit expected is a style, you can specify multiple values */
+    text-style: strike bold italic reverse;
+    text-style: bold underline italic;
+}
+```
+
+```py
+widget.styles.text_style = "bold"
+widget.styles.text_style = "italic"
+widget.styles.text_style = "reverse"
+widget.styles.text_style = "underline"
+widget.styles.text_style = "strike"
+
+# Multiple values can be specified
+widget.styles.text_style = "strike bold italic reverse"
+widget.styles.text_style = "bold underline italic"
+```
+
+## Used by
+
+ - Links:
+    - [`link-style`](../links/link_style.md)
+    - [`link-hover-style`](../links/link_hover_style.md)
+ - [`text-style`](../text_style.md)

docs/styles/snippets/text_style_syntax.md

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+The [text style](/styles/css_units/text_style) unit can be any space-separated combination of the following values:
+
+| Value       | Description                                                    |
+|-------------|----------------------------------------------------------------|
+| `bold`      | **bold text**                                                  |
+| `italic`    | _italic text_                                                  |
+| `reverse`   | reverse video text (foreground and background colors reversed) |
+| `underline` | <u>underline text</u>                                          |
+| `strike`    | <s>strikethrough text</s>                                      |