HID: core: Improve the kerneldoc for hid_report_len()

The kerneldoc for hid_report_len() needs to be improved.  The
description of the @report argument is ungrammatical, and the
documentation does not explain under what circumstances the report
length will include the byte reserved for the report ID.

Let's fix up the kerneldoc.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://patch.msgid.link/1c8416cb-7347-4a06-b00a-20518069d263@rowland.harvard.edu


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>