include/hw/boards.h: Document memory_region_allocate_system_memory()

Add a documentation comment for memory_region_allocate_system_memory().

In particular, the reason for this function's existence and the
requirement on board code to call it exactly once are non-obvious.

Backports commit 09ad643823dcda0a86eddce1291c28d0ccb09a3b from qemu

qemu/include/hw/boards.h

@@ -25,6 +25,34 @@ struct QEMUMachine {
     int minimum_page_bits;
 };
 
+/**
+ * memory_region_allocate_system_memory - Allocate a board's main memory
+ * @mr: the #MemoryRegion to be initialized
+ * @owner: the object that tracks the region's reference count
+ * @name: name of the memory region
+ * @ram_size: size of the region in bytes
+ *
+ * This function allocates the main memory for a board model, and
+ * initializes @mr appropriately. It also arranges for the memory
+ * to be migrated (by calling vmstate_register_ram_global()).
+ *
+ * Memory allocated via this function will be backed with the memory
+ * backend the user provided using "-mem-path" or "-numa node,memdev=..."
+ * if appropriate; this is typically used to cause host huge pages to be
+ * used. This function should therefore be called by a board exactly once,
+ * for the primary or largest RAM area it implements.
+ *
+ * For boards where the major RAM is split into two parts in the memory
+ * map, you can deal with this by calling memory_region_allocate_system_memory()
+ * once to get a MemoryRegion with enough RAM for both parts, and then
+ * creating alias MemoryRegions via memory_region_init_alias() which
+ * alias into different parts of the RAM MemoryRegion and can be mapped
+ * into the memory map in the appropriate places.
+ *
+ * Smaller pieces of memory (display RAM, static RAMs, etc) don't need
+ * to be backed via the -mem-path memory backend and can simply
+ * be created via memory_region_init_ram().
+ */
 void memory_region_allocate_system_memory(MemoryRegion *mr, Object *owner,
                                           const char *name,
                                           uint64_t ram_size);