fix(ui): close floating window on BufLeave event (#16557)

When buffer is visible in two splits simultaneously, BufHidden event is
not triggered, causing the floating window to remain on screen after
switching to another buffer.

Remove BufHidden event from close_events defaults, and close the window
if we changed the buffer to something other than the buffer that spawned
the floating window or the floating window buffer itself.

runtime/doc/lsp.txt

@@ -1377,18 +1377,6 @@ character_offset({bufnr}, {row}, {col})
                     (number, number) UTF-32 and UTF-16 index of the character
                     in line {row} column {col} in buffer {buf}
 
-                                        *vim.lsp.util.close_preview_autocmd()*
-close_preview_autocmd({events}, {winnr})
-                Creates autocommands to close a preview window when events
-                happen.
-
-                Parameters: ~
-                    {events}  (table) list of events
-                    {winnr}   (number) window id of preview window
-
-                See also: ~
-                    |autocmd-events|
-
                               *vim.lsp.util.convert_input_to_markdown_lines()*
 convert_input_to_markdown_lines({input}, {contents})
                 Converts any of `MarkedString` | `MarkedString[]` |