fs_mgr: Change scratch margin to 3 X logical_block_size.

Found a device with a 3/4MB erase block, give a wider margin so that
flashing a single partition is unlikely to be blocked by scratch.

Add documentation to tell developers to flashall to clear scratch.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: Ic219283f4c42e457b98991bcd1752253e182eff3