FCHashMap code readability and documentation improvement

I recently spent time reviewing PR: https://github.com/swirlds/swirlds-platform/pull/6447
Without diagrams and documentation it was very hard to read to make sense of.

Anybody who needs to understand that code would have a very hard time and either will have to reach out to the author or as most developers do just be brave and spend a lot of time reading it.

Based on personal experience
1. A few simple diagrams preferably embedded to the javadoc
2. Naming variables as close to the problem they solve, though this is always difficult to get it right
3. Add succinct (tricky) documentation at the right(tricky) places