Update docs for --configPlugin using typescript

[Jimmydalecleveland]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

The documentation gives examples for using @rollup/plugin-typescript to parse the Rollup configuration file, with a CLI code example. However, that example will fail to build with a somewhat misleading error if the Rollup config is not declared in the tsconfig.json's include property, so a note about needing to do so has been added with this commit.

For an example of the error I'm referring to, here is what happened to me:

[!] Error: Unexpected token (Note that you need plugins to import files that are not JavaScript)
rollup.config.ts (5:13)
3: import del from "rollup-plugin-delete";
4:
5: const options: RollupOptions = {
                ^
6:   input: "src/index.ts",
7:   output: [
Error: Unexpected token (Note that you need plugins to import files that are not JavaScript)

This is pretty obvious when you know the answer, but when you first get this error after following the docs, it is a bit confusing because it seems like something is wrong with your CLI flags or the config itself due to the error giving the same output as if the @rollup/plugin-typescript was not setup at all.

I'm happy to reword this, or move it somewhere else if anyone has suggestions.

[codecov]

Codecov Report

Merging #4495 (bd5ca1a) into master (8c6e0f3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4495   +/-   ##
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  Coverage   98.72%   98.72%           
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  Files         206      206           
  Lines        7351     7351           
  Branches     2086     2086           
=======================================
  Hits         7257     7257           
  Misses         34       34           
  Partials       60       60           

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