ext4: update journal documentation

ext4: update journal documentation

Add a section about journal checkpointing, including information about
the ioctl EXT4_IOC_CHECKPOINT which can be used to trigger a journal
checkpoint from userspace.

Also, update the journal allocation information to reflect that up to
10240000 blocks are used for the journal and that the journal is not
necessarily contiguous.

Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
Changes in v5:
- clarify behavior of DRY_RUN flag
Link: https://lore.kernel.org/r/20210518151327.130198-3-leah.rumancik@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>