Document changes and release 0.20.0~beta3

There's some things that would be nice to have

• workig bits of !1083 (closed)
• !1103 (merged)
• maybe !1104 (merged)
• maybe !1106 (merged)
• !1107 (merged)
• !1108 (merged)

and since we want to release past phoc 0.21.0 let's prepare for another beta.