ext4: Log inline_data feature in Documentation

When I tested inline_data feature, I found it hasn't been documented in ext4.txt.

---

index f7cbf57..f2de1a1 100644
@@ -99,6 +99,7 @@ Note: More extensive information for getting started with ext4 can be

 * large block (up to pagesize) support
 * efficient new ordered mode in JBD2 and ext4(avoid using buffer head to force
   the ordering)
+* allows small files or directories to be stored within the in-inode extended attribute area via inline_data
 [1] Filesystems with a block size of 1k may see a limit imposed by the
 directory hash tree having a maximum depth of two.