add description about text blocks (#1028)

[Lyther]

Hi Remkop,
For issue #1028 , I've added descriptions about java 15's text block in user manual in both html and adoc files.
This may save your time of writing document.
Enderaoe

[remkop]

Hi @Lyther, thank you!
I was hoping to include a working example in the documentation.
Would it be possible for you to provide a working example that you have tested on Java 15?

[Lyther]

Hi @remkop
I've added an example in description and tested under Java 15, it works fine.
The user manual would look like this now:
That's a pleasure to contribute to this awesome project.
Enderaoe

[remkop]

Merged into master.
Thank you for the contribution!
I will update the release-notes later.

[remkop]

FYI:
https://twitter.com/picocli/status/1258371028876582912
Thanks!