Document limited number of characters in FontFile

[radarhere]

Helps #5124

When saving, FontFile only saves 256 characters.

Pillow/src/PIL/FontFile.py

Lines 102 to 111 in b48cfa7

	with open(os.path.splitext(filename)[0] + ".pil", "wb") as fp:
	fp.write(b"PILfont\n")
	fp.write(f";;;;;;{self.ysize};\n".encode("ascii")) # HACK!!!
	fp.write(b"DATA\n")
	for id in range(256):
	m = self.metrics[id]
	if not m:
	puti16(fp, [0] * 10)
	else:
	puti16(fp, m[0] + m[1] + m[2])

This PR notes this when discussing converting from other font formats.