doc: tweak guidance for modules in core

doc/guides/components-in-core.md

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+# To be or not to be in core
+
+This document explains things to consider when deciding whether a component
+should be in core or not.
+
+A component may be included in core as a dependency, a module, or integrated
+into the code base. The same arguments for including/not including in core
+generally apply in all of these cases.
+
+## Strong arguments for including a component in core
+
+1. The component provides functionality that is standardized (such as a
+   [Web API][]) and overlaps with existing functionality.
+2. The component can only be implemented in core.
+3. The component can only be implemented in a performant way in core.
+4. Developer experience is significantly improved if the component is in core.
+5. The component provides functionality that can be expected to solve at
+   least one common use case Node.js users face.
+6. The component requires native bindings. Inclusion in core enables
+   utility across operating systems and architectures without requiring
+   users to have a native compilation toolchain.
+7. Part or all of the component will also be re-used or duplicated in core.
+
+## Strong arguments against including a component in core
+
+1. None of the arguments listed in the previous section apply.
+2. The component has a license that prohibits Node.js from including it in core
+   without also changing its own license.
+3. There is already similar functionality in core and adding the component will
+   provide a second API to do the same thing.
+4. A component (or/and the standard it is based on) is deprecated and there is
+   a non-deprecated alternative.
+5. The component is evolving quickly and inclusion in core will require frequent
+   API changes.
+
+## Benefits and challenges
+
+When it is unclear whether a component should be included in core, it might be
+helpful to consider these additional factors.
+
+### Benefits
+
+1. The component will receive more frequent testing with Node.js CI and CITGM.
+2. The component will be integrated into the LTS workflow.
+3. Documentation will be integrated with core.
+4. There is no dependency on npm.
+
+### Challenges
+
+1. Inclusion in core, rather than as an ecosystem module, is likely to reduce
+   code merging velocity. The Node.js process for code review and merging is
+   more time-consuming than that of most separate modules.
+2. By being bound to the Node.js release cycle, it is harder and slower to
+   publish patches.
+3. Less flexibility for users. They can't update the component
+   when they choose without also updating Node.js.
+
+[Web API]: https://developer.mozilla.org/en-US/docs/Web/API