[image_picker_for_web] Add doc comments to point out that some arguments are'nt supported on the web

[devj3ns]

The arguments maxWidth, maxHeight and imageQuality of pickImage() and the argument maxDuration of pickVideo() are not supported in web version of the image_picker package.

I added some doc comments to point out that specifying these argument does not change anything on the pickedFile.

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[ditman]

@devj3ns this is great! however because of how the code is imported, these code blocks might not be discoverable.

Do you mind adding a "Limitations of the web version" to the README of the package (similar to this, or this), so the information is right there in the front page of pub?

Thanks!!

[devj3ns]

Hey @ditman, I added the information to the README.md file :)

[ditman]

Fixed a little typo, let's go with this one!

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[ditman]

I broke this PR, fixing :)