Unclear which "variables" `iterate_over` can be applied to

### Page link:

https://bazel.build/docs/cc-toolchain-config-reference#flag-groups

### Problem description (include actual vs expected text, if applicable):

The docs say:

> For flags that need to repeat with different values when added to the build command, the flag group can iterate variables of type list

But this is not true. It can only iterate over certain magic list variables (which are not documented), not _any_ list variable. Certainly it cannot iterate over user-defined variables.

It would be good if the docs could clarify exactly which list variables `iterate_over` can be applied to.

Even better would be if `iterate_over` could be applied also to user-defined lists, not only these "magic" lists.

### Where do you see this issue? (include link to specific section of the page, if applicable)

https://bazel.build/docs/cc-toolchain-config-reference#flag-groups

### Any other information you'd like to share?

_No response_

Start with the `cc-toolchain-config-reference#flag-groups` section and trace the definition of `iterate_over` and the list variables it supports. Done means the documentation explicitly names the supported variables, distinguishes them from user-defined lists, and clearly states whether user-defined-list support is out of scope.

Written by the indexing model from the issue text.