Update docs: Understanding turbo.json

[Tittoh]

Update docs to show updated error messages.
I created a new project and quick search on a similar error brought me here.

run failed: error preparing engine: Could not find the following tasks in project: hello
Turbo error: error preparing engine: Could not find the following tasks in project: hello

according to the docs, Understanding turbo.json the expected error is:

task `hello` not found in turbo `pipeline` in "turbo.json".
Are you sure you added it?

dev, lint and `build commands are working as expected.

v: 1.8.3

Originally posted by @Tittoh in #3918 (comment)