Improve typecheck docs

[bombsimon]

Add a step in the troubleshooting guide to ensure users are not linting individual files. Even though this might work, it's often the root cause of typecheck reports when the package consists of multiple files.

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This seems to be a common mistake and mentioned in #3759. This step ensures users targeting single files tries to lint the whole package first (and not necessarily ./...).

[ldez]

The sentence is ambiguous because:

• A single file or package can be linted if there are no dependencies with other files/packages
• package has the same "problem" as a single file, if there are dependencies with other files/packages it will fail.

Your suggestion is the same topic as Ensure building works with go run ./.../go build ./... - whole package.

I think I will create PR to explain with more details what is typecheck, it will not be fun but I think it's the best approach we can take.

[bombsimon]

Yes, the sentence wasn't meant to be an absolute truth for every time typecheck is seen, it was just one thing to try in the troubleshooting process. Same as go mod tidy won't always solve the issue but it might solve the issue and worth having the user test.

I think it would be nice to extend the list to ensure that golangci-lint is not ran on a single file since that's pretty common mistake (either with the CLI tool or with editor settings). However you're explanation in #4306 is better so I'll close this.