btrfs: update outdated comment for overwrite_item()

The function is exclusively used for log replay since commit
3eb4234 ("btrfs: remove outdated logic from overwrite_item() and add
assertion"), so update the comment so that it doesn't say it can be used
for logging. Also some minor rewording for clarity and while at it
reformat the affected text so that it fits closer to the 80 characters
limit for comments.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/tree-log.c

@@ -376,12 +376,12 @@ static int process_one_buffer(struct btrfs_root *log,
 }
 
 /*
- * Item overwrite used by replay and tree logging.  eb, slot and key all refer
- * to the src data we are copying out.
+ * Item overwrite used by log replay. The given eb, slot and key all refer to
+ * the source data we are copying out.
  *
- * root is the tree we are copying into, and path is a scratch
- * path for use in this function (it should be released on entry and
- * will be released on exit).
+ * The given root is for the tree we are copying into, and path is a scratch
+ * path for use in this function (it should be released on entry and will be
+ * released on exit).
  *
  * If the key is already in the destination tree the existing item is
  * overwritten.  If the existing item isn't big enough, it is extended.