Doc: document max raw dev number

Documenting description about max minor number of raw devices.

Signed-off-by: Kazuo Moriwaka <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

Documentation/devices.txt

···
 		  1 = /dev/raw/raw1	First raw I/O device
 		  2 = /dev/raw/raw2	Second raw I/O device
 		    ...
+                 max minor number of raw device is set by kernel config
+                 MAX_RAW_DEVS or raw module parameter 'max_raw_devs'
 
 163 char