docs/memory.txt: Fix document on MMIO operations

.impl.valid should be .impl.unaligned and the description needs some
fixes.

.old_portio is removed since commit b40acf99b (ioport: Switch
dispatching to memory core layer).

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

docs/memory.txt

@@ -232,8 +232,8 @@ various constraints can be supplied to control how these callbacks are called:
    (in bytes) supported by the *implementation*; other access sizes will be
    emulated using the ones available.  For example a 4-byte write will be
    emulated using four 1-byte writes, if .impl.max_access_size = 1.
- - .impl.valid specifies that the *implementation* only supports unaligned
-   accesses; unaligned accesses will be emulated by two aligned accesses.
- - .old_portio and .old_mmio can be used to ease porting from code using
-   cpu_register_io_memory() and register_ioport().  They should not be used
-   in new code.
+ - .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
+   cpu_register_io_memory(). It should not be used in new code.

include/exec/memory.h

@@ -110,7 +110,7 @@ struct MemoryRegionOps {
         /* If true, unaligned accesses are supported.  Otherwise all accesses
          * are converted to (possibly multiple) naturally aligned accesses.
          */
-         bool unaligned;
+        bool unaligned;
     } impl;
 
     /* If .read and .write are not present, old_mmio may be used for