Clean RELEASE-NOTES for 2.2

I kept the notes on Python 3 incompatibility and the advice to
compile with Qt 4.8.x.
Make style of RELEASE-NOTES consistent
C-a is now bound to inset-select-all

C-M-a is bound to the global "select all". The
reason for this change is that selecting an inset
is a more common operation that selecting an entire
document.

This patch is the result of discussion on #7727.
RELEASE-NOTES: Describe obsoletion of \\rtl