mm: add pagemap.h to the fine documentation

The documentation currently does not include the deathless prose written
to describe functions in pagemap.h because it's not included in any rst
file.  Fix up the mismatches between parameter names and the documentation
and add the file to mm-api.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: http://lkml.kernel.org/r/20200221220045.24989-1-willy@infradead.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/mm-api.rst

@@ -73,6 +73,9 @@ File Mapping and Page Cache
 .. kernel-doc:: mm/truncate.c
    :export:
 
+.. kernel-doc:: include/linux/pagemap.h
+   :internal:
+
 Memory pools
 ============
 

include/linux/pagemap.h

@@ -33,8 +33,8 @@ enum mapping_flags {
 
 /**
  * mapping_set_error - record a writeback error in the address_space
- * @mapping - the mapping in which an error should be set
- * @error - the error to set in the mapping
+ * @mapping: the mapping in which an error should be set
+ * @error: the error to set in the mapping
  *
  * When writeback fails in some way, we must record that error so that
  * userspace can be informed when fsync and the like are called.  We endeavor
@@ -303,9 +303,9 @@ static inline struct page *find_lock_page(struct address_space *mapping,
  * atomic allocation!
  */
 static inline struct page *find_or_create_page(struct address_space *mapping,
-					pgoff_t offset, gfp_t gfp_mask)
+					pgoff_t index, gfp_t gfp_mask)
 {
-	return pagecache_get_page(mapping, offset,
+	return pagecache_get_page(mapping, index,
 					FGP_LOCK|FGP_ACCESSED|FGP_CREAT,
 					gfp_mask);
 }