Documentation: f2fs: Reword title

"What is F2FS" is rather a mistitle for the whole f2fs docs, as it
implies the overview section (before "Background and design issues"
section) and the docs covers beyond that: from mount options to
filesystem implementation details.

Retitle and add explicit overview section.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: bagasme

Documentation/filesystems/f2fs.rst

@@ -1,8 +1,11 @@
 .. SPDX-License-Identifier: GPL-2.0
 
-==========================================
-WHAT IS Flash-Friendly File System (F2FS)?
-==========================================
+=================================
+Flash-Friendly File System (F2FS)
+=================================
+
+Overview
+========
 
 NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
 been equipped on a variety systems ranging from mobile to server systems. Since