Brief note about origin of Lena image

[aarchiba]

No description provided.

[jaimefrio]

Missing a preposition (is "in" the right one here?) between "work" and "most".

[rgommers]

Good idea to add a note on this. Maybe it would also make sense to say something like:

This image has historically been used a lot for image processing
examples/applications and is kept for backwards compatibility. However for 
new code that needs an example image, we recommend to use `ascent` or `face`
instead of `lena`.

Also, I see now that ascent and face are not listed in misc/__init__.py or in the See Also of lena and therefore quite invisible. Could you fix that as well in this PR?

EDIT: IIRC ascent and face were added exactly to provide a better alternative to using lena

[rgommers]

Squashed and merged in d9b221f with a very minor tweak (``` around function names in docstring instead of ' to create links, and prepend `DOC:` to commit message). Thanks @aarchiba!