add text styles docs

doc/en/text-styles.md

@@ -0,0 +1,21 @@
+# Text styles
+
+A proprietary Markdown dialect is used to mark up text styles.
+Formatting works on UI elements: label and textbox, if `markdown="true"` is explicitly specified.
+
+## Styles
+
+| Style         | Example                  | Output                     |
+| ------------- | ------------------------ | -------------------------- |
+| Bold          | `**Bold font**`          | **Bold font**              |
+| Italic        | `*Text in italics*`      | *Text in italics*          |
+| Underline     | `__Underlined text__`    | <ins>Underlined text</ins> |
+| Strikethrough | `~~Strikethrough text~~` | ~~Strikethrough text~~     |
+
+Styles can be combined. Example:
+```md
+***__Message__*** using *~~combed~~ combined* styles__~~.~~__
+```
+Output:
+
+***<ins>Message</ins>*** using *~~combed~~ combined* styles<ins>~~.~~</ins>