Explicitly mention the "..=" compound assignment operator everywhere
".=" is documented.
Convert some uses of "." and ".=" in the examples to ".." and "..=",
respectively.
closes: vim/vim#18380b6f9d38f96
Cherry-pick a change to eval.txt from patch 9.1.1232.
Co-authored-by: Doug Kearns <dougkearns@gmail.com>