bug#2058: 23.0.60; bewildering behavior when visit new file throw.pdf

Hi Alan & Drew,

I've made some changes:

  - I enhanced the docstring to mention what doc-view actually is.

  - If the file doesn't exist or is empty, then we fallback to ps-mode
    or fundamental-mode, depending on the file name extension

  - If the images cannot be displayed (no png support, missing
    conversion utility, tty frame), then the message is more explicit.
    Now it says to which mode C-c C-c would switch, the C-c C-t message
    was enhanced according to Alan's suggestion and it mentions `k'
    which kills the buffer and stops a possibly running conversion.

Please check if those changes DTRT, or if something is still too crappy
to be released with 23.1.

Bye,
Tassilo