Is there documentation for [buildenv] in profiles?

I'm doing work on a project where I'm trying to make things as Conan 2.0 as possible, using `CMakeToolchain` and the new `VirtualBuildEnv`.

We have path variables that I figured out have to go into `[buildenv]`, and that the syntax for prepending to the path is `=+(path)`, like this compatible profile example.

```
include(llvm-toolset-7)
[env]
PATH=[/opt/cmake-3.19/bin]
[buildenv]
PATH=+(path)/opt/cmake-3.19/bin
```

I figured this out _from the code_.

Now that `PATH=+(path)/opt/cmake-3.19/bin` syntax looks familiar to me, and I thought I saw it somewhere.

For the life of me, I can't find doc on this. Am I missing something?