rust: fix doc comments that trigger rust warnings

Rust generates warnings that are treated as errors for documentation
blocks before `extern` blocks.

(cherry picked from commit 6ccc01a79c)

rust/src/core.rs

@@ -188,7 +188,7 @@ pub fn sc_app_layer_decoder_events_free_events(
 /// Opaque flow type (defined in C)
 pub enum Flow {}
 
-/// Extern functions operating on Flow.
+// Extern functions operating on Flow.
 extern {
     pub fn FlowGetLastTimeAsParts(flow: &Flow, secs: *mut u64, usecs: *mut u64);
 }

rust/src/json.rs

@@ -24,7 +24,7 @@ use std::os::raw::c_char;
 /// The Rust place holder for the json_t pointer.
 pub enum JsonT {}
 
-/// Expose the jansson functions we need.
+// Expose the jansson functions we need.
 extern {
     fn json_object() -> *mut JsonT;
     fn json_object_set_new(js: *mut JsonT, key: *const c_char,