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 <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Miguel Ojeda <[e-mail]>
Reviewed-by: Sui Jingfeng <[e-mail]>
Cc: Helge Deller <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/video/fbdev/core/fb_info.c

···
  *
  * Creates a new frame buffer info structure. Also reserves @size bytes
  * for driver private data (info->par). info->par (if any) will be
- * aligned to sizeof(long).
+ * aligned to sizeof(long). The new instances of struct fb_info and
+ * the driver private data are both cleared to zero.
  *
  * Returns the new structure, or NULL if an error occurred.
  *