[#57] Upgradeable contract doc

Description

Just looking at an upgradeable contract it is hard to understand how it works, especially if you care not only about the interface but also about implementation.

Now the upgradeable contract has doc which more or less descriptive.

Apart from adding documentation for all upgradeable contract entrypoints, storage type documentation was added.

As a side change, documentEntryPoint was added. This function provides an ability to document single entrypoint without unwrapping its parameter type (entryCase unconditionally performs unwrapping)

Related issue(s)

Resolves part of #57 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • README
    • Haddock
    • docs/
  • I updated the changelog if my changes are externally visible.

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