Fix nits.

src/expressions.md

@@ -89,10 +89,12 @@ rvalue expression to a `'static` slot occurs when the expression could be
 written in a constant, borrowed, and dereferencing that borrow where the
 expression was the originally written, without changing the runtime behavior.
 That is, the promoted expression can be evaluated at compile-time and the
-resulting value does not contain interior mutability or destructors.
+resulting value does not contain interior mutability or destructors (these
+properties are determined based on the value where possible, e.g. `&None`
+always has the type `&'static Option<_>`, as it contains nothing disallowed).
 Otherwise, lifetime of temporary values is typically the innermost enclosing
 statement; the tail expression of a block is considered part of the statement
-that encloses the block
+that encloses the block.
 
 When a temporary rvalue is being created that is assigned into a `let`
 declaration, however, the temporary is created with the lifetime of the