## Description

This PR adds more documentation for `TextEditingController(String text)` constructor and it adds one example.

https://github.com/flutter/flutter/pull/96245 was a first improvement to the documentation.
https://github.com/flutter/flutter/issues/79495 tried to hide the cursor when an invalid selection is set but it was reverted.
https://github.com/flutter/flutter/pull/123777 mitigated the issue of having a default invalid selection: it takes care of setting a proper selection when a text field is focused and its controller selection is not initialized.

I will try changing the initial selection in another PR, but It will probably break several existing tests.

## Related Issue

Fixes https://github.com/flutter/flutter/issues/95978

## Tests

Adds 1 test for the new example.