Fix and clarify description of self.folders.subproject

[aagor]

The old documentation was wrong, as the subproject is not required to be the folder, where the conanfile.py is located.
An example, where this is not the case, can be found here: https://docs.conan.io/2/examples/conanfile/layout/conanfile_in_subfolder.html

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[aagor]

I'm open for suggestions (or direct edits), if you have better wording or want to clarify further.

[memsharded]

While this is technically correct, I am not sure this was the original intention and design of the feature. The self.folders.subproject intends to have the conanfile.py in that location, and then the rest of the relative things of the subproject relative to it.
That is, the idea is that the conanfile.py is found at the root of the subproject. Maybe it doesn't work exactly like that for all cases, but for example, the docs in https://docs.conan.io/2/examples/conanfile/layout/multiple_subprojects.html show this usage.

It looks like we probably want to investigate the implications of this, if there could be some problems of explicitly documenting and encouraging this layout.

[aagor]

Well, I don't say or recommend this practice here of putting the conanfile.py inside a subfolder.

Where I can imagine this is used, is for open-source dependencies that ship upstream Conan/vcpkg/Debian/etc. support and don't want to clutter the main directory with files specific to one package manager.

But the point is, when the conanfile.py is in a subfolder, it`s usually wrong and problematic to set the subproject to this folder.