doc/go1.19: complete most remaining TODOs

The ones I left behind are almost entirely ones that I see pending CLs for.
Also make various fixes to existing text.

For #51400.

Change-Id: I555e0074c9df82b5bdb345e21a08c8757ca147b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/410814
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>