kernel/vm: Update/add some documentation comments and drop some old files.

They mostly contained descriptions of functions' operations, not
what they're for, so they weren't so useful. I've added some of
the remarks they had (or ones with a similar purpose) inline
as documentation comments, and deleted the rest.

headers/private/system/vm_defs.h

@@ -41,6 +41,9 @@ enum {
 enum {
 	// TODO: these are here only temporarily - it's a private
 	// addition to the BeOS create_area() lock flags
+	//! (Only for use during the boot process.) Indicates the specified address
+	//! is already mapped, and the corresponding pages should be found and
+	//! inserted into the new cache.
 	B_ALREADY_WIRED	= 7,
 };