docs/devel/memory.txt: Add section about RAM migration

the whole address range; this is often clearer and is preferred.

Subregions cannot be added to an alias region.

Migration

---------

Where the memory region is backed by host memory (RAM, ROM and

ROM device memory region types), this host memory needs to be

copied to the destination on migration. These APIs which allocate

the host memory for you will also register the memory so it is

migrated:

 - memory_region_init_ram()

 - memory_region_init_rom()

 - memory_region_init_rom_device()

For most devices and boards this is the correct thing. If you

have a special case where you need to manage the migration of

the backing memory yourself, you can call the functions:

 - memory_region_init_ram_nomigrate()

 - memory_region_init_rom_nomigrate()

 - memory_region_init_rom_device_nomigrate()

which only initialize the MemoryRegion and leave handling

migration to the caller.

The functions:

 - memory_region_init_resizeable_ram()

 - memory_region_init_ram_from_file()

 - memory_region_init_ram_from_fd()

 - memory_region_init_ram_ptr()

 - memory_region_init_ram_device_ptr()

are for special cases only, and so they do not automatically

register the backing memory for migration; the caller must

manage migration if necessary.

Region names

------------