fbdev: Document that framebuffer_alloc() returns zero'ed data

Most fbdev drivers depend on framebuffer_alloc() to initialize the
allocated memory to 0. Document this guarantee.

v3:
	* slightly reword the sentence (Miguel)

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Cc: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230715185343.7193-19-tzimmermann@suse.de