usb: gadget: f_fs: Document eventfd effect on descriptor format.

When FUNCTIONFS_EVENTFD flag is set, __ffs_data_got_descs reads a 32bits,
little-endian value right after the fixed structure header, and passes it
to eventfd_ctx_fdget. Document this.

Also, rephrase a comment to be affirmative about the role of string
descriptor at index 0. Ref: USB 2.0 spec paragraph "9.6.7 String", and
also checked to still be current in USB 3.0 spec paragraph "9.6.9 String".

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>