Improve documentation of -Cjump-tables

Be more verbose about what this option can and cannot do.

Author: pmur

src/doc/rustc/src/codegen-options/index.md

@@ -212,15 +212,23 @@ with coverage tools other than those built and shipped with the compiler.
 ## jump-tables
 
 This option is used to allow or prevent the LLVM codegen backend from creating
-jump tables when lowering switches.
+jump tables when lowering switches from Rust code.
 
 * `y`, `yes`, `on`, `true` or no value: allow jump tables (the default).
 * `n`, `no`, `off` or `false`: disable jump tables.
 
+To prevent jump tables being created from Rust code, a target must ensure
+all crates are compiled with jump tables disabled.
+
+Note, in many cases the rust toolchain is distributed with precompiled
+crates, such as the core and std crates, which could possibly include
+jump tables. Furthermore, this option does not guarantee a target will
+be free of jump tables. They could arise from external dependencies,
+inline asm, or other complicated interactions when using crates which
+are compiled with jump table support.
+
 Disabling jump tables can be used to help provide protection against
-jump-oriented-programming (JOP) attacks. However, this option makes
-no guarantee any precompiled or external dependencies are compiled
-with or without jump tables.
+jump-oriented-programming (JOP) attacks.
 
 ## link-arg