cleanup: reformat sentence about max sizes

The extent size must fits all blocks in 4294967295 sectors
(in 512b units) this is 1/2 KiB less then 2TiB.

So while previous statement 'suggested' 2TiB is still acceptable value,
make it clear it's not.

As now we support any multiples of 128KB as extent size -
values like 2047G will still 'flow-in' otherwise the largest power-of-2
supported value is 1TiB.

With 1TiB user needs 8388608 extents for 8EiB device.
(FYI such device is already unusable with todays glibc-2.22.90-27)

4GiB extent size is currently the smallest extent size which allows
a user to create 8EiB devices (with 2GiB  it's less then 8EiB).

TODO: lvm2 may possibly print amount of 'lost/unused space' on a PV,
since using such ridiculously sized extent size may result in huge
space being left unaccessible.

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