Add paragraph about not discussing effectiveness

Author: rcvalle

src/doc/rustc/src/exploit-mitigations.md

@@ -33,6 +33,11 @@ mitigations in order to mitigate vulnerabilities resulting from the use of
 Unsafe Rust. This chapter is going to document these exploit mitigations and
 how they apply to Rust.
 
+This chapter does not discuss the effectiveness of these exploit mitigations
+as they vary greatly depending on several factors besides their design and
+implementation, but rather describe what they do, so their effectiveness can
+be understood within a given context.
+
 
 ## Exploit mitigations