docs: add more details about ya.sync() and its usage

docs/plugins/overview.md

@@ -121,7 +121,6 @@ You can also obtain a small amount of app data from the sync context by calling
 
 ```lua
 -- ~/.config/yazi/plugins/my-async-plugin.yazi/init.lua
-
 local get_hovered_url = ya.sync(function(a, b)
 	-- You can access all app data through the `cx`,
 	-- within the `sync()` block, in an async plugin
@@ -138,6 +137,18 @@ return {
 }
 ```
 
+Note that `ya.sync()` call must be at the top level, and do not use it in sync plugins, as these are undefined behaviors that may lead to unexpected results:
+
+```lua
+-- Wrong !!!
+local get_hovered_url
+if some_condition then
+	get_hovered_url = ya.sync(function(a, b)
+		-- ...
+	end)
+end
+```
+
 ## Interface
 
 ### Previewer

docs/plugins/utils.md

@@ -94,7 +94,7 @@ Append messages to [the log file](./overview.md#logging) at the error level:
 
 ### `sync(fn)`
 
-TODO
+See [Async context](/docs/plugins/overview.md#async-context).
 
 ### `preview_code(opts)`