docs: cgroup-v1: Update page cache removal functions

Commit 452e9e6992fe ("filemap: Add filemap_remove_folio and
__filemap_remove_folio") reimplemented __delete_from_page_cache() as
__filemap_remove_folio() and delete_from_page_cache() as
filemap_remove_folio(). The compatibility wrappers were finally removed
in ece62684dcfb ("hugetlbfs: convert hugetlb_delete_from_page_cache() to
use folios") and 6ffcd825e7d0 ("mm: Remove __delete_from_page_cache()").

Update the remaining references to dead functions in the memcg
implementation memo.

Signed-off-by: Illia Ostapyshyn <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

Documentation/admin-guide/cgroup-v1/memcg_test.rst

···
 	The logic is very clear. (About migration, see below)
 
 	Note:
-	  __remove_from_page_cache() is called by remove_from_page_cache()
+	  __filemap_remove_folio() is called by filemap_remove_folio()
 	  and __remove_mapping().
 
 6. Shmem(tmpfs) Page Cache