docs: Update the function section of the tutorial for Dafny 4

[turbotimon]

Description

• In the tutorial there still Dafny v3.0 explanation and exercise regarding function method.
• I changed the text and exercise 5 to new v4.0 ghost function syntax.
• Text was mostly copied from Dafny Documentation
• Please feel free to change my suggestion as you wish

How has this been tested?

• Text: Spellcheck
• Code: Dafny VSCode Extension

👉️ fixes #5007

[fabiomadge]

Thanks for your contribution! The problem with the paragraph you added is, that ghost functions aren't necessarily opaque, which is what the test describes.
I tried converting the functions to ghost functions, but came to the conclusion that this is an unnecessary distraction at that point of the reader's learning journey, now that we changed the default.
Let me know what you think.

[turbotimon]

Hi @fabiomadge, that also works for me! Whatever you think is best :)