Note additional things as unsafe

src/items/external-blocks.md

@@ -101,9 +101,14 @@ specified name.
 extern { }
 ```
 
-The type of a function declared in an extern block is `extern "abi" fn(A1, ...,
-An) -> R`, where `A1...An` are the declared types of its arguments and `R` is
-the declared return type.
+A function declared in an extern block is implicitly `unsafe`. When coerced to
+a function pointer, a function declared in an extern block has type `unsafe
+extern "abi" for<'l1, ..., 'lm> fn(A1, ..., An) -> R`, where `'l1`, ... `'lm`
+are its lifetime parameters, `A1`, ..., `An` are the declared types of its
+parameters and `R` is the declared return type.
+
+It is `unsafe` to access a static item declared in an extern block, whether or
+not it's mutable.
 
 It is valid to add the `link` attribute on an empty extern block. You can use
 this to satisfy the linking requirements of extern blocks elsewhere in your

src/unsafety.md

@@ -7,14 +7,14 @@ The following language level features cannot be used in the safe subset of
 Rust:
 
 - Dereferencing a [raw pointer].
-- Reading or writing a [mutable static variable].
-- Reading a field of a [`union`], or writing to a field of a
-  union that isn't [`Copy`].
+- Reading or writing a [mutable] or [external] static variable.
+- Accessing a field of a [`union`], other than to assign to it.
 - Calling an unsafe function (including an intrinsic or foreign function).
 - Implementing an [unsafe trait].
 
 [`Copy`]: special-types-and-traits.html#copy
 [`union`]: items/unions.html
 [mutable static variable]: items/static-items.html#mutable-statics
+[external static variable]: items/external-blocks.html
 [raw pointer]: types.html#pointer-types
 [unsafe trait]: items/traits.html#unsafe-traits