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ARROW-8909: [Java] Out of order writes using setSafe

As explicit asked by the customer, we should make it clear in the document that variable width vectors do not support out of order writes.

This could avoid unnecessary confusion.

Closes #7354 from liyafan82/fly_0605_doc

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>