Fix and improve the document for Explicit Nulls

[ShapelessCat]

• Fix some text and adjust the markdown format.
• Insert one space before and after | to make the code examples have a consistent style.
• Adjust the link to the image "Type Hierarchy for Explicit Nulls": put this image name in the [] part instead of putting it after the hyperlink text.
• Convert "UnsafeNulls" to a in-file link that points to the subsection ###UnsafeNulls
• Update tests/explicit-nulls/neg/flow-varref-in-closure.scala to tests/explicit-nulls/neg/flow-varref-in-closure.scala, and also convert it to a link that points to file ../../../../tests/explicit-nulls/neg/flow-varref-in-closure.scala.
• Remove ()s from some side-effect free Scala method calls.

[ShapelessCat]

I have a question to this line:

val d2: Array[String | Null] = d1 // unsafely convert Array[String] to Array[String | Null]

which is the line 492 before changing, and line 508 after changing.

I didn't touch this line, but I don't quite understand the "unsafely" in its comment.
Since d1 is of type Array[String], it seems val d2: Array[String | Null] = d1 doesn't include any unsafe operation. Can anyone explain to me why this assignment is unsafe. Thanks!

[noti0na1]

Thanks for the fix!

Because Array in Scala is invariant, Array[String] is not a subtype of Array[String | Null]. Hence, this conversion is unsafe.