add explanation of concepts to various design.md documents

There is a mismatch (see tables, below) between the concepts attributed to an exercise in [csharp/reference/README.md](https://github.com/exercism/v3/blob/master/languages/csharp/reference/README.md) which reflects the original concept-scraping task performed by the track's maintainers and what is shown for each exercise in [csharp/config.json](https://github.com/exercism/v3/blob/master/languages/csharp/config.json) which is the official catalog of completed exercises.

The task, here, is to align the concepts associated in an exercise in _csharp/config.json_ with those in _csharp/reference/exercises.json_ (from which _csharp/reference/README.md_ is generated).

For each concept found in _config.json_, not in _exercises.json_
- check if the concept is fully illustrated or described in the exercise to which it is attributed
- if the concept is fully illustrated or described then:
- add the concept's name and a brief description to the exercise's _design.md_ document
- add the concept to the exercise in _csharp/reference/exercises.json_.
- if the concept is not fully illustrated or described in the exercise but it is an appropriate concept for the exercise then:
- amend the exercise's _introduction.md_, _after.md_ and/or the code files and instructions to reflect the concept
- add the concept's name and a brief description to the exercise's _design.md_ document
- add the concept to the exercise in _csharp/reference/exercises.json_.
- if the concept is not fully illustrated or described in the exercise and it is not an appropriate concept for the exercise then:
- add the concept to the _unallocated-concepts_ exercise at the end of _exercises.json_ where it can be picked up for further review

For each concept found in _exercises.json_, not in _config.json_
- check if the concept is fully illustrated or described in the exercise to which it is attributed
- if the concept if fully illustrated or described then:
- add the concept to the exercise in _csharp/config.json_.
- if the concept is not fully illustrated or described in the exercise but it is an appropriate concept for the exercise then:
- amend the exercise's _introduction.md_, _after.md_ and/or the code files and instructions to reflect the concept
- add the concept to the exercise in _csharp/config.json_.
- if the concept is not fully illustrated or described in the exercise and it is not an appropriate concept for the exercise then:
- take the concept from the exercise in _exercises.json_ and move it to the _unallocated-concepts_ exercise in _exercises.json_ where it can be picked up for further review

### Concepts in csharp/config.json not in csharp/reference/exercises.json

| Exercise | Concept |
|----------|---------|
| flag-enums | compound-assignment |
| integral-numbers | casting |
| method-overloading | named-arguments |

### Concepts in csharp/reference/exercises.json not in csharp/config.json

| Exercise | Concept |
|----------|---------|
| classes | encapsulation |
| classes | fields |
| classes | objects |
| classes | state |
| classes | visibility |
| classes | default-value |
| inheritance | polymorphism |
| basics | return-values |
| basics | mutation |
| basics | functions |
| basics | type-inference |
| basics | comments |
| basics | scoping |
| basics | variables |
| nullability | null-coalescing |
| nullability | null-conditional |
| nullability | null-forgiving |
| nullability | nullable-values |
| numbers | math-operators |
| numbers | assignment |
| dictionaries | base-class-library |
| equality | marker-interfaces |
| parameters | named-arguments |
| parameters | out-parameters |
| parameters | optional-arguments |
| time | cross-platform |
| interfaces | explicit-interfaces |
| integral-numbers | unsigned-integers |
| lists | collections |
| namespaces | imports |
| string-formatting | interpolation |
| casting | implicit-casting |