Update documentation about use of NATVIS

The docs were still talking about Visual Studio 2013, which we've not
supported in a while.

I made the documentation a bit more vague because NATVIS is sometimes
automatically picked up when building and sometimes requires manually
installing the files. (For me, LLVM NATVIS is automatically picked up
while Clang NATVIS needs manual installation; this happens because the
way the monorepo is organized causes confusion when used with Visual
Studio's built-in CMake functionality. Building from a .sln file
generated by CMake may have better behavior in this case.)

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