Note to Windows users that FreeType will keep the font file open

[radarhere]

Resolves #6324

It has been requested we document that on Windows, FreeType keeps the font file open while Pillow is using it.

Testing this,

import shutil
shutil.copy("Tests/fonts/FreeMono.ttf", "out.ttf")
font = ImageFont.truetype("out.ttf")
os.remove("out.ttf")

on Windows, I see a PermissionError. On macOS and Linux, there is no problem.

The error disappears on CPython if you insert del font before os.remove.