mm/doc: Add documentation for folio_test_uptodate

Move the PG_uptodate documentation to be documentation for
folio_test_uptodate() and expand on it a little.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>

include/linux/page-flags.h

@@ -68,9 +68,6 @@
  * might lose their PG_swapbacked flag when they simply can be dropped (e.g. as
  * a result of MADV_FREE).
  *
- * PG_uptodate tells whether the page's contents is valid.  When a read
- * completes, the page becomes uptodate, unless a disk I/O error happened.
- *
  * PG_referenced, PG_reclaim are used for page reclaim for anonymous and
  * file-backed pagecache (see mm/vmscan.c).
  *
@@ -615,6 +612,16 @@ TESTPAGEFLAG_FALSE(Ksm, ksm)
 
 u64 stable_page_flags(struct page *page);
 
+/**
+ * folio_test_uptodate - Is this folio up to date?
+ * @folio: The folio.
+ *
+ * The uptodate flag is set on a folio when every byte in the folio is
+ * at least as new as the corresponding bytes on storage.  Anonymous
+ * and CoW folios are always uptodate.  If the folio is not uptodate,
+ * some of the bytes in it may be; see the is_partially_uptodate()
+ * address_space operation.
+ */
 static inline bool folio_test_uptodate(struct folio *folio)
 {
 	bool ret = test_bit(PG_uptodate, folio_flags(folio, 0));