doc: update syntax description of immutability

"Stored by value" is more correct way to explain that example.

Mutable vs immutable means that you cannot mutate the value (e.g. list vs tuple in Python), and the example shows that `var2` is actually mutable.

Copying/storing a reference vs value is what what matters in the assignment, in Python `a=b` means `a` and `b` are references to the same list, while in meson `a=b` copies the value of `b` into `a`.

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