Doc string of editorconfig-indent-size-vars needs some improvements

[Eli Zaretskii]

It is quite obscure:

  (defvar editorconfig-indent-size-vars
    #'editorconfig--default-indent-size-function
    "Rule to use to set a given `indent_size'.
  Can take the form of a function, in which case we call it with a single SIZE
  argument (an integer) and it should return a list of (VAR . VAL) pairs.
  Otherwise it can be a list of symbols (those which should be set to SIZE).
  Major modes are expected to set this buffer-locally.")

It should explain how will the VAR/VAL pairs be used for indentation.
It should also tell what is SIZE (one could guess, but why should one?).

Thanks.