Image.open() seeks to the start of file objects

[radarhere]

Resolves #7096 by documenting that Image.open() seeks to the start of file objects before reading.

Pillow/src/PIL/Image.py

Lines 3202 to 3225 in aec7a8d

	try:
	fp.seek(0)
	except (AttributeError, io.UnsupportedOperation):
	fp = io.BytesIO(fp.read())
	exclusive_fp = True
	prefix = fp.read(16)
	preinit()
	accept_warnings = []
	def _open_core(fp, filename, prefix, formats):
	for i in formats:
	i = i.upper()
	if i not in OPEN:
	init()
	try:
	factory, accept = OPEN[i]
	result = not accept or accept(prefix)
	if type(result) in [str, bytes]:
	accept_warnings.append(result)
	elif result:
	fp.seek(0)