doc: update for new_thread note

[erw7]

There seems to be a problem if the luv_thread_t userdata is released before the thread terminates. I have confirmed that the thread entry function may not be executed because thd->code was released by gc as shown below by the debugger. I have not been successful in creating a test case. An example of a segmentation fault in luv_thread_arg_clear, possibly due to this issue, has been reported in the neovim issue(neovim/neovim/issues/22694).

test.lua

local uv = require'luv'

uv.new_thread(function()
  print('Hello')
end)
collectgarbage('collect')

-- wait for thread to finish
uv.sleep(3000)

> gdb luajit
…
>>> break luv_thread_gc
…
>>> commands
>p "call luv_thread_gc"
>cont
>end
>>> break luv_thread_cb
…
>>> commands
>call sleep(1)
>p "call luv_thread_cb"
>p ((luv_thread_t*)varg)->code
>p ((luv_thread_t*)varg)->len
>cont
>end
>>> run /home/erw7/test.lua
…
Thread 1 "luajit" hit Breakpoint 1, luv_thread_gc (L=0x7fffff3f0380) at ../src/thread.c:253
253     static int luv_thread_gc(lua_State* L) {
$1 = "call luv_thread_gc"
…
Thread 2 "luajit" hit Breakpoint 2, luv_thread_cb (varg=0x7fffff40d178) at ../src/thread.c:269
269     static void luv_thread_cb(void* varg) {
$2 = 0
$3 = "call luv_thread_cb"
$4 = 0x0
$5 = 0
…