spec: clarify "slice of bytes" and "slice of runes" through examples

spec: clarify "slice of bytes" and "slice of runes" through examples

The spec section on conversions uses the terms "slice of bytes" and
"slice of runes". While not obviously clear, what is meant are slice
types whose element types are byte or rune types; specifically the
underlying types of the slices' element types must be byte or rune.

Some of this was evident from the examples, but not all of it. Made
this clearer by adding more examples illustrating various permitted
conversions.

Note that the 1.17 compiler did not accept the following conversions:

        string([]myByte{...})
        string([]myRune{...})
        myString([]myByte{...})
        myString([]myRune{...})

(where myByte, myRune, and myString have underlying types of byte,
rune, and string respectively) - it reported an internal error.
But it did accept the inverse conversions:

        []myByte("...")
        []myRune("...")
        []myByte(myString("..."))
        []myRune(myString("..."))

The 1.18 compiler made those conversions symmetric and they are now
permitted in both directions.

The extra examples reflect this reality.

Fixes #23814.

Change-Id: I5a1c200b45ddd0e8c0dc0d11da3a6c39cb2dc848
Reviewed-on: https://go-review.googlesource.com/c/go/+/412094
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>