Add documentation on meaning of -EPROBE_DEFER

Add a bit of documentation on what it means when a driver .probe() hook
returns the -EPROBE_DEFER error code, including the limitation that
-EPROBE_DEFER should be returned as early as possible, before the driver
starts to register child devices.

Also: minor markup fixes in the same file

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Saravana Kannan <saravanak@google.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Grant Likely <grant.likely@arm.com>
Link: https://lore.kernel.org/r/20200327170132.17275-1-grant.likely@arm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>