DOCS: IMDG Reference Manual: “What is HZ IMDG” - minor improvements

https://github.com/hazelcast/imdg-docs/blob/master/docs/modules/overview/pages/what-is-imdg.adoc

1. Hazelcast IMDG is highly scalable and available. - > it sounds like it is generally available and hight scalable, I think “Highly available” is kinda fixed phrase, so, I would propose to change to “highly scalable and highly available”
2. offering distributed implementations of many developer-friendly interfaces -> Not clear for me what are “developer-friendly interfaces”. It would be better to change to something like “of most popular map and collection interfaces” - this will be natively understandable by Java devs.
3. Key Value store which confers many of the advantages of NoSQL in terms of developer friendliness and developer productivity - > As for me, it is a bit complex sentence, it’s better to simplify, say, “a developer friendly In-memory NoSQL Key value store ”
4. It is flexible enough -> I think it’s better to use more positive sentence like “It is so flexible that out-of-box can be used as…”
5. it is delivered as a compact library (JAR) - > it seems JAR isa really well-known term, so, for most Java devs, will be more convenient sentence like “as a JAR file (Java library archive)”
6. a member in the cluster crashes as data -> such wording implies our members can crash periodically, I think it’s better to rephrase like “a member become unavailable”
7. Hazelcast is a fit when you need - > it seems, more “advertisement” should be where, propose to change to “HZ is the best when you need:”
8. an alternative to Coherence and Terracotta -> Not totally sure, but competitors names here means that everybody who is reading this, will understand thy are the option and will try to compare. Probably, it could be better to describe this without names, like “powerful distributed in-memory data storage”