[vim/vim] Refined autoload mechanism explanation (PR #20833)

Based on the discussion in https://github.com/vim/vim/issues/19205#issuecomment-5065053347, I took the freedom of clarifying an autoload related paragraph that read a bit too cryptic. 

I also wanted to add a note by which Section `*52.4*	Other mechanisms to use` only applied to legacy Vim.

In-fact, I tried something similar in Vim9 through the following script:

```vim
vim9script noclear

# Note that if you define var did_load inside the if-endif block, then the scope of did_load is confined inside the if-endif block.
export var did_load = false
echom "Initial did_load: " .. did_load

if !did_load
  command -nargs=* BNRead BufNetRead(<f-args>)
  map <F19> :call BufNetWrite('something')<CR>

  did_load = true
  echom "Final did_load: " .. did_load
  exe 'au FuncUndefined BufNet* source ' .. expand('<sfile>')
  finish
endif

def BufNetRead(a: string)
  echo 'BufNetRead(' .. string(a) .. ')'
  # read defality here
enddef

def BufNetWrite(a: string)
  echo 'BufNetWrite(' .. string(a) .. ')'
  # write defality here
enddef
```

and I expected to see

```vim
false
true
true
```

But I got:

```vim
false
true
false
true
```

which suggests that what is suggested there only apply to legacy vim script.
You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/20833

-- Commit Summary --

  * Refined autoload mechanism explanation

-- File Changes --

    M runtime/doc/usr_52.txt (129)

-- Patch Links --

https://github.com/vim/vim/pull/20833.patch
https://github.com/vim/vim/pull/20833.diff

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