Update Istio differences topic

https://docs.openshift.com/container-platform/4.7/service_mesh/v2x/ossm-vs-community.html

OSSMDOC-328 addressed some formatting in this topic, but reading through it, I think it needs a deeper review than I can get to. 

It looks to me like it was in response to customer feedback that customers wanted to hear about the differences between Istio and OSSM. The first question that came to mind is that this assembly hasn't changed much in a couple of years. Has that customer desire changed?

Second, is this the right place for this kind of information? Could this information be better distributed and maintained as marketing material or a techdoc? Do these kinds of questions come up when reading through the docs, or at the sales call stage or marketing level? 

If the answer to all of that is that we should keep this assembly, it could benefit from a few updates.

• The first and second paragraph should be reworked to describe more about what the topic is about. 
• Many of the headings, for example Multitennant, seem to assume knowledge of Istio and start with a description of what Istio does followed by what OSSM does. Consider flipping that around. Instead of leading with Istio, lead with features of OSSM and how Istio differs with it. 
• Some headings, like Installation and upgrades, just say that the istio items are not supported. For the items that are unsupported, consider making an unsupported section and list the items that are not supported.