update whatsnew/ migration

update whatsnew/ migration

The migration20 guide had a lot of "whats new" stuff in it,
which is moved to the whatsnew20 document.

Also moved most of the content regarding the reflection rewrite
into the whatsnew20 document and added some performance comparisons
to 1.4.

The sectioning for these two documents, as has happened for some
other documents I'm working on, has been modified to only use the
double-equals operator at the top.  This is strictly because my
vscode seems to be able to show me an .rst outline in the side panel
if I do it this way, and otherwise it's blank.

Change-Id: I1c89af8b6fd47a3c14f33becc6f9ee06b1743a0f