help.vim's syntax

Hi everybody,

I have understood that for vim's help, the syntax file used is help.vim
(in the directory /usr/share/vim/vim80/syntax).

I can use it with:
:so help.vim
(if it is in the same directory of the file i open)

But i still don't understand why, when i open one file in
/usr/share/vim/vim80/doc
EVEN A NEW FILE
it automatically uses the syntax of help.vim, whereas i do nothing...
Maybe someone can explain me.

Best regards,
Mathieu Roux

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