[en/rust] Emphasize the difference between &str and String.

The difference between a heap allocated `String` and a Slice `&str` is not emphasized properly.
As already mentioned a `String` is a heap allocated String. In addition it should be clearly emphasized, that a `String` is a vector of bytes and always holds a valid UTF-8 sequence. Its growable and does not hold a null termination character!

When it comes to `&str` I miss a comment about the fact, that slices are allocates in read-only-memory. The association between a heap allocated string and a slice needs to be explained in detail:

A slice is a `&[u8]` and points to a valid UTF-8 sequence, and can be used to view into a String. A `&[T] ` is a view into `Vec`.

Especially for a Rust-beginner these side notes can be very useful for a deeper understanding.

Start by opening the en/rust document and reading its existing String and &str sections. Revise the explanation to clearly distinguish ownership, UTF-8 validity, growth, termination, and the relationship between strings, byte slices, and vectors; it is done when these beginner-focused differences are accurate and explicit.

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