Improve documentation and add more uses of MBEDTLS_PUT

minor changes, such as improving the documentation for the byte reading
macros, and using MBEDTLS_PUT_UINT16_xy in place of byte reading
macro combinations

Signed-off-by: Joe Subbiani <joe.subbiani@arm.com>
diff --git a/library/common.h b/library/common.h
index fb4194e..c35bc04 100644
--- a/library/common.h
+++ b/library/common.h
@@ -61,9 +61,8 @@
 
 /** Byte Reading Macros
  *
- * Obtain the most significant byte of x using 0xff
- * Using MBEDTLS_BYTE_a will shift a*8 bits
- * to retrieve the next byte of information
+ * Given a multi-byte integer \p x, MBEDTLS_BYTE_n retrieves the n-th 
+ * byte from x, where byte 0 is the least significant byte.
  */
 #define MBEDTLS_BYTE_0( x ) ( (uint8_t) (   ( x )         & 0xff ) )
 #define MBEDTLS_BYTE_1( x ) ( (uint8_t) ( ( ( x ) >> 8  ) & 0xff ) )