doc/memory.txt: correct description of MemoryRegionOps fields

Probably what happened was that when the API was being designed it
started off with an 'aligned' field, and then later the field name
and semantics were changed but the docs weren't updated to match.

Similarly, cpu_register_io_memory() does not exist anymore, so
clarify the documentation for .old_mmio.

Reported-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/memory.txt

@@ -297,8 +297,9 @@ 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
    have device and bus specific behaviour (ignored, or machine check)
- - .valid.aligned specifies that the device only accepts naturally aligned
-   accesses.  Unaligned accesses invoke device and bus specific behaviour.
+ - .valid.unaligned specifies that the *device being modelled* supports
+    unaligned accesses; if false, unaligned accesses will invoke the
+    appropriate bus or CPU specific behaviour.
  - .impl.min_access_size, .impl.max_access_size define the access sizes
    (in bytes) supported by the *implementation*; other access sizes will be
    emulated using the ones available.  For example a 4-byte write will be
@@ -306,5 +307,5 @@ various constraints can be supplied to control how these callbacks are called:
  - .impl.unaligned specifies that the *implementation* supports unaligned
    accesses; if false, unaligned accesses will be emulated by two aligned
    accesses.
- - .old_mmio can be used to ease porting from code using
+ - .old_mmio eases the porting of code that was formerly using
    cpu_register_io_memory(). It should not be used in new code.