kernel/vm: Clarify locking of VMArea and drop old documentation.

The documentation file hadn't been updated in a very long time,
and referred to locks and paradigms that have long since been
dropped (e.g. sMappingLock was removed in 2010, in
afed2658f4ef3e3df5ff8390d1d7a51b5b897a9e.)

Add a few "protected by" comments in VMArea to clarify things,
but otherwise this document looks entirely obsolete (I went
through all of it.)

headers/private/kernel/vm/VMArea.h

@@ -112,11 +112,11 @@ private:
 	uint16					memory_type;	// >> shifted by MEMORY_TYPE_SHIFT
 
 public:
-	VMCache*				cache;
+	VMCache*				cache;			// protected by sAreaCacheLock
 	off_t					cache_offset;
 	uint32					cache_type;
-	VMAreaMappings			mappings;
-	uint8*					page_protections;
+	VMAreaMappings			mappings;		// protected by the TranslationMap lock
+	uint8*					page_protections; // protected by cache lock
 
 	struct VMAddressSpace*	address_space;