Add descriptions for impls in the blocks module

[keckler]

What is the change?

Adding longer implementation descriptions for the blocks module.

Why is the change being made?

Quality.

---

Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any important changes.
• The documentation is still up-to-date in the doc folder.
• If any requirements were affected, mention it in the release notes.
• The dependencies are still up-to-date in pyproject.toml.

[albeanth]

will reopen this when it's ready for review. Following this rule for @john-science https://github.com/terrapower/armi/blob/main/doc/developer/tooling.rst#dont-open-until-it-is-ready

[john-science]

@keckler Closed? Is there a problem I should know about?

[albeanth]

@keckler Closed? Is there a problem I should know about?

@john-science #1619 (comment)

[jyang-TP]

Review has finished.