lucet-runtime: fix doc comment about C context
//!
//! let module = DlModule::load("/my/lucet/module.so").unwrap();
//! let region = MmapRegion::create(1, &Limits::default()).unwrap();
-//! struct PretendLibcRuntime { _unused: u8 };
-//! let mut libc = PretendLibcRuntime { _unused: 0 };
+//! #[repr(C)]
+//! struct MyForeignContext { _unused: u8 };
+//! let mut foreign_ctx = Box::into_raw(Box::new(MyForeignContext{ _unused: 0 }));
//! let mut inst = region
//! .new_instance_builder(module)
-//! .with_embed_ctx(libc)
+//! .with_embed_ctx(foreign_ctx as *mut libc::c_void)
//! .build()
//! .unwrap();
//!
//!
//! // clean up embedder context
//! drop(inst);
+//! // foreign_ctx must outlive inst, but then must be turned back into a box
+//! // in order to drop.
+//! unsafe { Box::from_raw(foreign_ctx) };
//! ```
//!
//! ## Custom Signal Handlers
