docs: render @since versions, 0 means experimental #30649

An implication of this current approach is that `NVIM_API_LEVEL` should be
bumped when a new Lua function is added.

TODO(future): add a lint check which requires `@since` on all new functions.

ref #25416

runtime/lua/vim/_meta/builtin.lua

@@ -248,7 +248,7 @@ function vim.schedule(fn) end
 ---     - If {callback} errors, the error is raised.
 function vim.wait(time, callback, interval, fast_only) end
 
---- Attach to ui events, similar to |nvim_ui_attach()| but receive events
+--- Attach to |ui-events|, similar to |nvim_ui_attach()| but receive events
 --- as Lua callback. Can be used to implement screen elements like
 --- popupmenu or message handling in Lua.
 ---
@@ -282,6 +282,8 @@ function vim.wait(time, callback, interval, fast_only) end
 --- end)
 --- ```
 ---
+--- @since 0
+---
 --- @param ns integer
 --- @param options table<string, any>
 --- @param callback fun()