Documentation: spi-nor: rewrite some portions

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>

Documentation/mtd/spi-nor.txt

                           SPI NOR framework
                ============================================
 
-Part I - why we need this framework?
--------------------------------------
+Part I - Why do we need this framework?
+---------------------------------------
 
-The SPI bus controller only deals with the byte stream.
-Some controller does not works like a SPI bus controller, it works
-like a SPI NOR controller instead, such as the Freescale's QuadSPI controller.
+SPI bus controllers (drivers/spi/) only deal with streams of bytes; the bus
+controller operates agnostic of the specific device attached. However, some
+controllers (such as Freescale's QuadSPI controller) cannot easily handle
+arbitrary streams of bytes, but rather are designed specifically for SPI NOR.
 
-The Freescale's QuadSPI controller should know the NOR commands to
-find the right LUT sequence. Unfortunately, the old code can not meet
-this requirement.
+In particular, Freescale's QuadSPI controller must know the NOR commands to
+find the right LUT sequence. Unfortunately, the SPI subsystem has no notion of
+opcodes, addresses, or data payloads; a SPI controller simply knows to send or
+receive bytes (Tx and Rx). Therefore, we must define a new layering scheme under
+which the controller driver is aware of the opcodes, addressing, and other
+details of the SPI NOR protocol.
 
 Part II - How does the framework work?
--------------------------------------
+--------------------------------------
 
 This framework just adds a new layer between the MTD and the SPI bus driver.
 With this new layer, the SPI NOR controller driver does not depend on the
@@ -40,7 +44,7 @@ m25p80 code anymore.
          ------------------------
 	       SPI NOR chip
 
-  With the SPI NOR controller driver(Freescale QuadSPI), it looks like:
+  With the SPI NOR controller driver (Freescale QuadSPI), it looks like:
                    MTD
          ------------------------
               SPI NOR framework
@@ -49,11 +53,10 @@ m25p80 code anymore.
          ------------------------
 	       SPI NOR chip
 
-Part III - How can the drivers use the framework
--------------------------------------
+Part III - How can drivers use the framework?
+---------------------------------------------
 
-The main API is the spi_nor_scan(). Before you call the hook, you should
-initialize the necessary fields for spi_nor{}.
-Please see the drivers/mtd/spi-nor/spi-nor.c for detail.
-Please also reference to the fsl-quadspi.c when you want to write a new driver
-for a SPI NOR controller.
+The main API is spi_nor_scan(). Before you call the hook, a driver should
+initialize the necessary fields for spi_nor{}. Please see
+drivers/mtd/spi-nor/spi-nor.c for detail. Please also refer to fsl-quadspi.c
+when you want to write a new driver for a SPI NOR controller.