docs/devel/memory.txt: Document _with_attrs accessors

When we added the _with_attrs accessors we forgot to mention
them in the documentation.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-Id: <20180824170422.5783-4-peter.maydell@linaro.org>
Based-on: <20180802174042.29234-1-peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

docs/devel/memory.txt

@@ -326,8 +326,15 @@ visible as the pci-hole alias clips it to a 0.5GB range.
 MMIO Operations
 ---------------
 
-MMIO regions are provided with ->read() and ->write() callbacks; in addition
-various constraints can be supplied to control how these callbacks are called:
+MMIO regions are provided with ->read() and ->write() callbacks,
+which are sufficient for most devices. Some devices change behaviour
+based on the attributes used for the memory transaction, or need
+to be able to respond that the access should provoke a bus error
+rather than completing successfully; those devices can use the
+->read_with_attrs() and ->write_with_attrs() callbacks instead.
+
+In addition various constraints can be supplied to control how these
+callbacks are called:
 
  - .valid.min_access_size, .valid.max_access_size define the access sizes
    (in bytes) which the device accepts; accesses outside this range will