Improved image file formats documentation

[radarhere]

Addresses concerns raised in #6273 (reply in thread)

1. https://pillow.readthedocs.io/en/latest/handbook/image-file-formats.html#gif

The library writes LZW encoded files in GIF87a by default, unless GIF89a features are used or GIF89a is already in use.

This could be interpreted to mean that LZW encoding is not used for GIF89a. This is not the case (and @raygard would know, as he actually wrote #5291).

2. The "Reading local images" example relies on the ability of the user to set im.size. This was removed in Changed Image size property to be read-only by default #3203, so I've updated the example to use crop() instead.

3. https://pillow.readthedocs.io/en/latest/handbook/image-file-formats.html

So if copy() is called on an image, or another method internally creates a copy of the image, the fp (file pointer), along with any methods and attributes specific to a format. The format attribute will be None.

The first sentence seems to just stop, so I've reworked it.

[raygard]

Thanks! This looks much better. The "read local image" example works now. I didn't realize you could do it like that; it may come in handy.