doc: use "abstract type" terminology consistently

[nsajko]

Sometimes when the docs say "abstract", it means a type declared with abstract type. In other occasions, it refers to any type that's not concrete. For clarity and consistency, I propose that all instances of the latter usage be replaced, e.g., with "non-concrete".

xref #48142

Examples:

• The Union doc string says: "Like other abstract types"
• The Tuple doc string says: "Tuple{Any} is considered an abstract type"