Current "variables in import statements" limitations.

Specifically #2772.

Probably it would be helpful to document that @import statement is evaluated "right away" before anything else (specifically mixins). Or, in other words, that it's a sort of "static" statement and it's evaluated exactly where it appears. E.g. when we write something like:

.mixin() {
    @import "something";
}

the code is literally equal to:

.mixin() {
    <the content of the "something" file>
}

thus mixin parameters cannot be interpolated in such import statements, because the import is unconditionally expanded/resolved at the point of the mixin definition (and not when we call/invode that mixin) when and where the mixin parameters values are yet unknown.

Start by reading issue #2772 and the existing documentation for Less @import statements. Document that imports are evaluated at their definition point, before mixin parameters are known, and include the example showing why parameter interpolation cannot work; done means the limitation and its timing are clear to users.