mm: correct the docs for thp_fault_alloc and thp_fault_fallback

The documentation does not align with the code.  In
__do_huge_pmd_anonymous_page(), THP_FAULT_FALLBACK is incremented when
mem_cgroup_charge() fails, despite the allocation succeeding, whereas
THP_FAULT_ALLOC is only incremented after a successful charge.

Link: https://lkml.kernel.org/r/20240412114858.407208-5-21cnbao@gmail.com
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/transhuge.rst

@@ -369,7 +369,7 @@ monitor how successfully the system is providing huge pages for use.
 
 thp_fault_alloc
 	is incremented every time a huge page is successfully
-	allocated to handle a page fault.
+	allocated and charged to handle a page fault.
 
 thp_collapse_alloc
 	is incremented by khugepaged when it has found
@@ -377,7 +377,7 @@ thp_collapse_alloc
 	successfully allocated a new huge page to store the data.
 
 thp_fault_fallback
-	is incremented if a page fault fails to allocate
+	is incremented if a page fault fails to allocate or charge
 	a huge page and instead falls back to using small pages.
 
 thp_fault_fallback_charge