Improve TextArea doc

[glebfox]

Existing page: https://docs.jmix.io/jmix/flow-ui/vc/components/textArea.html

1. Worth mention that the input TextArea, unlike TextField , can only be of type String.
2. Worth mention that unless set to a fixed height, Text Area adjusts its height automatically based on its content. The default and minimum height is two rows of text.
3. Add css, ariaLabel, ariaLabelBy, focusShortcut common attributes
4. Don't use px in code samples. Use em instead. By default 1em = 16px, so 300px can be replaced with 20em (aka 320px)
5. Add Style variants description and examples: See Vaadin doc