chapter4: Variable storage description is inprecise

The text in chapter 4 contains:

    Firmware needs a method to modify variable storage at runtime while
    the OS controls access to the device."

We explicitly don't require SetVariable() at runtime. But this sentence
sounds normative. We should stick to RFC 2119 and add MAY here.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

Author: xypron

source/chapter4-firmware-media.rst

@@ -35,7 +35,7 @@ conflict with normal usage of the media by an OS.
 * Firmware must be stored on the media in a way that does not conflict
   with normal partitioning and usage by the operating system.
 * Normal operation of the OS must not interfere with firmware files.
-* Firmware needs a method to modify variable storage at runtime while the
+* Firmware may need a method to modify variable storage at runtime while the
   OS controls access to the device. [#LUVariables]_
 
 .. [#LUVariables] Runtime access to firmware data may still be an issue when