f2fs: add description about small_discards in document
f2fs: add description about small_discards in document

This patch adds a description about small_disacrds in the f2fs document.

Signed-off-by: Jaegeuk Kim <20424effba9d4aeeb410130c5f23aa328fa29e09@samsung.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs