Modify instruction text on custom header screen

[jane]

Current text:
"You can upload a custom header image to be shown at the top of your site instead of the default one. On the next screen you will be able to crop the image."

Issue:
If the theme in question does not come with default header images, this text is confusing. We wrote it with 2010/2011 in mind, but it's confusing with other themes (especially since the Custom Header Preview now shows the blog name and tagline text).

Suggested text change:
"You can select an image to be shown at the top of your site by uploading from your computer or choosing from your Media Library. After selecting an image you will be able to crop it if desired."

This also does a better job of setting up the recent addition of choosing header images from the library.

This one appears to be good to go.

In [21502]:

Better instruction text on custom header screen. Props jane, JustinSainton fixes #20992