Update documentation of function copy_type_recursive

Update documentation of function copy_type_recursive

Following commit 8f57eec2fb3 ("Use gdbarch obstack to allocate types in
alloc_type_arch") it is no longer the case that the type returned by
copy_type_recursive is allocated using malloc.  Because the function
uses alloc_type_arch internally, the new type is now allocated on the
gdbarch associated with the type, and is thus owned by that gdbarch.

gdb/ChangeLog:

	* gdbtypes.c (copy_type_recursive): Update documentation.