Improved putdata() documentation and data handling

[radarhere]

Resolves #5858

1. Clarify that putdata() should receive a flattened sequence, rather than the 2D sequence that the user is trying in the issue.

I have drawn inspiration from the docstring for getdata()

Pillow/src/PIL/Image.py

Lines 1277 to 1282 in 11c536b

	def getdata(self, band=None):
	"""
	Returns the contents of this image as a sequence object
	containing pixel values. The sequence object is flattened, so
	that values for line one follow directly after the values of
	line zero, and so on.

2. Raise an error if the sequence is not flattened.

>>> from PIL import Image
>>> im = Image.new("L", (1, 1), 0)
>>> im.putdata([2.0])
>>> print(im.getpixel((0, 0)))

At the moment, in Python 3.9, this returns 2, and in Python 3.10, this returns 0. I've added a commit here to consistently return 2.