bug#15627: 23.4; docstring for `mode-line-format' should link to default

[Glenn Morris]

Samuel Bronson wrote:

> Basically, this is a suggestion to include "`default-mode-line-format'"
> somewhere in the docstring, preferably near the top.

Why does it need to be mentioned at all?

C-h v default-mode-line-format

  This variable is obsolete since 23.2;
  use `mode-line-format' instead.

Documentation:
Default value of `mode-line-format' for buffers that don't override it.
This is the same as (default-value 'mode-line-format).