Document CR mapping and workarounds

[xdg]

The CR wrapping isn't well documented, particularly how it handles existing mappings. I had problems with a pumvisible expr mapping but found a workaround moving the expression into a function and using a <C-R>=...<CR> mapping instead, which composes correctly with vim-closer. E.g.

function! HandleCRKey() abort
  return pumvisible() ? "\<C-E>\n" : "\n"
endfunction

" vim-closer can extend this correctly
inoremap <silent> <CR> <C-R>=HandleCRKey()<CR>