docs/manual: add allowed notations section for genimage.cfg files

Add allowed notations section for genimage.cfg files by specifying the
accepted suffix or prefix. Note that "K" has been decided to specify
KiloBytes as 1024 bytes. This has been done because in Buildroot the 90%
of the cases were using "K" and not "k".

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/writing-rules.txt

@@ -192,6 +192,12 @@ image sdimage.img {
 * Filename must at least begin with genimage prefix and have the .cfg
   extension to be easy to recognize.
 
+* Allowed notations for +offset+ and +size+ options are: +G+, +M+, +K+
+  (not +k+).  If it's not possible to express a precise byte count
+  with notations above then use hexadecimal +0x+ prefix or, as last
+  chance, the byte count.  In comments instead use +GB+, +MB+, +KB+
+  (not +kb+) in place of +G+, +M+, +K+.
+
 The +genimage.cfg+ files are the input for the genimage tool used in
 Buildroot to generate the final image file(i.e. sdcard.img). For further
 details about the _genimage_ language, refer to