Update PHP Driver Documentation: 3.4 Feature Decimal128

If your driver does not have a playbook, updates for the 3.4 feature decimal128 will still be handled by the driver engineer.

The docs team will be working on writing the updates for node.js/ruby/java drivers. If you want to wait until those are done, you can then borrow the text from those and tweak the wording as appropriate for your driver and swap out examples.

Note: For decimal, for the actual release in november, am thinking of a tutorial for monetary values.