Improved image lifecycle documentation

[radarhere]

Resolves #5772 by improving the image lifecycle documentation.

• Copied images are not associated with the original file
• __exit__ does not destroy the core image object
  

  Pillow/src/PIL/Image.py

  Lines 579 to 585 in fa53b71

  	def __exit__(self, *args):
  	if hasattr(self, "fp") and getattr(self, "_exclusive_fp", False):
  	if hasattr(self, "_close__fp"):
  	self._close__fp()
  	if self.fp:
  	self.fp.close()
  	self.fp = None

[jenstroeger]

Great, thank you!