on-screen-keyboard: Add some clarifications based on feedback

This makes the following changes:

• Rewords some parts for clarity
• Mentions the Mildenhall widget and why it cannot trivially be repurposed
• Moves up some information on why Weston is not suitable as-is and links down to the "Recommendations" section, to serve as a highlight.

https://phabricator.apertis.org/T8305

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com