Fix a few notes within User Manual

[albeanth]

What is the change?

While reading parts of the UM, I've found a couple notes that are out-of-date.

1. There is no T/H module within ARMI so temperatures are not updated in ARMI with tight coupling.
2. Block heights don't have to be input at hot temperatures.

Why is the change being made?

Having correct docs is important.

---

Checklist

• [x] This PR has only one purpose or idea.
• ~Tests have been added/updated to verify any new/changed code.~

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

• ~The release notes have been updated if necessary.~
• [x] The documentation is still up-to-date in the doc folder.
• [x] The dependencies are still up-to-date in pyproject.toml.

[john-science]

@albeanth We can merge your PR, as-is, but perhaps we could also add this one-line change to the docs in this PR?

An error was noted in section 2.1.4.2 if the ARMI UM. The first line of the code block is

b = o.r.getFirstBlock(Flags.FUEL)

but should be:

b = o.r.core.getFirstBlock(Flags.FUEL)

At your discretion.

[albeanth]

At your discretion.

Done!