Better documentation of `:=` vs `<>`

**Type of issue**: documentation

**Impact**: no functional change

**Development Phase**: request

**Other information**

I feel like I have been in several conversations lately about how confusing `:=` and `<>` are. And I'm not even 100% confident that I could explain what each does in every scenario. This seems like something that a few Scastie examples + DecoupledIOs could "put to rest" and we could capture them as a cookbook or explanation on the website.

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

N/A

**What is the current behavior?**
N/A

**What is the expected behavior?**
N/A
**Please tell us about your environment:**

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**What is the use case for changing the behavior?**

Better documentation of an FAQ