Revise unsafety chapter for unsafe extern

In light of `unsafe extern`, we need to note that only unsafe extern
statics need `unsafe { .. }` and to clarify that not all foreign
functions are necessarily unsafe to call.

Co-authored-by: ehuss

Author: traviscross

src/unsafety.md

@@ -13,13 +13,13 @@ r[safety.unsafe-deref]
 - Dereferencing a [raw pointer].
 
 r[safety.unsafe-static]
-- Reading or writing a [mutable] or [external] static variable.
+- Reading or writing a [mutable] or unsafe [external] static variable.
 
 r[safety.unsafe-union-access]
 - Accessing a field of a [`union`], other than to assign to it.
 
 r[safety.unsafe-call]
-- Calling an unsafe function (including an intrinsic or foreign function).
+- Calling an unsafe function.
 
 r[safety.unsafe-target-feature-call]
 - Calling a safe function marked with a [`target_feature`][attributes.codegen.target_feature] from a function that does not have a `target_feature` attribute enabling the same features (see [attributes.codegen.target_feature.safety-restrictions]).