fix(messages)!: vim.ui_attach message callbacks are unsafe

Problem:  Lua callbacks for "msg_show" events with vim.ui_attach() are
          executed when it is not safe.
Solution: Disallow non-fast API calls for "msg_show" event callbacks.
          Automatically detach callback after excessive errors.
          Make sure fast APIs do not modify Nvim state.

runtime/doc/lua.txt

@@ -1083,6 +1083,10 @@ vim.ui_attach({ns}, {options}, {callback})                   *vim.ui_attach()*
     |ui-popupmenu| and the sections below for event format for respective
     events.
 
+    Callbacks for `msg_show` events are executed in |api-fast| context.
+
+    Excessive errors inside the callback will result in forced detachment.
+
     WARNING: This api is considered experimental. Usability will vary for
     different screen elements. In particular `ext_messages` behavior is
     subject to further changes and usability improvements. This is expected to

runtime/doc/news.txt

@@ -72,7 +72,8 @@ EDITOR
 
 EVENTS
 
-• TODO
+• |vim.ui_attach()| callbacks for |ui-messages| `msg_show` events are executed in
+  |api-fast| context.
 
 LSP
 

runtime/lua/vim/_meta/builtin.lua

@@ -233,6 +233,10 @@ function vim.wait(time, callback, interval, fast_only) end
 --- {callback} receives event name plus additional parameters. See |ui-popupmenu|
 --- and the sections below for event format for respective events.
 ---
+--- Callbacks for `msg_show` events are executed in |api-fast| context.
+---
+--- Excessive errors inside the callback will result in forced detachment.
+---
 --- WARNING: This api is considered experimental.  Usability will vary for
 --- different screen elements. In particular `ext_messages` behavior is subject
 --- to further changes and usability improvements.  This is expected to be