Clarify that maps and access to variables are optional

Signed-off-by: Dave Thaler <[email protected]>

isa/kernel.org/instruction-set.rst

@@ -517,11 +517,11 @@ where
 Map objects
 ~~~~~~~~~~~
 
-Maps are shared memory regions accessible by eBPF programs, where we use the term "map object"
+Maps are shared memory regions accessible by eBPF programs on some platforms, where we use the term "map object"
 to refer to an object containing the data and metadata (e.g., size) about the memory region.
 A map can have various semantics as defined in a separate document, and may or may not have a single
 contiguous memory region, but the 'mva(map)' is currently only defined for maps that do have a single
-contiguous memory region.
+contiguous memory region.  Support for maps is optional.
 
    **Note**
 
@@ -538,7 +538,7 @@ Variables
 
 Variables are memory regions, identified by integer ids, accessible by eBPF programs on
 some platforms.  The 'variable_addr(id)' operation means to get the address of the memory region
-identified by the given id.
+identified by the given id.  Support for such variables is optional.
 
    **Note**