Improve documentation of const (#26749)
* Improve documentation of const
Current description of `const` in the manual gave an impression that `const` ensures that variable binding may not change, e.g.:
> A common use of variables is giving names to specific, unchanging values. Such variables are only
assigned once. This intent can be conveyed to the compiler using the `const` keyword
and
> Note that `const` only affects the variable binding.
The proposed change describes the actual current behavior of `const`.
* additional example of const