Add note about editable package pitfall (#4749)

[onnodb]

This is a tiny update to the documentation, pointing out the small pitfall with editable packages I encountered in conan-io/conan#4749.

I hope this helps! Please do feel free to reject if you feel this is not a good place and/or wording.

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[danimtb]

I think this behavior is not specific to CMake, but any build system and working with editable packages

[danimtb]

I mean, this is the normal (maybe not expected at first try) behavior

[onnodb]

That's entirely fair, it's just that I offered to add a note in the documentation in the linked Conan issue.

I think it's enough of a potential trip-up that I figured it could save people troubleshooting time if one would point this out up-front. But feel free to close the PR if you don't think that's necessary.

[danimtb]

I think the note is fine, just make it more generic (it doesn't happen only with header libraries but any other library as well) and is not specific to CMake