docs(codecache): drop stale "approximate/#677" note now that memoryUsage is exact

With #677 merged, CodeCache::memoryUsage() is accurate on main, so the
runtime-stubs accessor comment describing it as an approximation (pending
#677) is no longer true. Describe current behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rkennke

ddprof-lib/src/main/cpp/hotspot/jitCodeCache.h

@@ -39,10 +39,9 @@ class JitCodeCache {
 
     static CodeBlob* findRuntimeStub(const void *address);
 
-    // Approximate heap usage of the runtime-stubs code cache, via
-    // CodeCache::memoryUsage() (a capacity/count-based estimate today; #677
-    // makes it exact). Read under the shared stubs lock because
-    // DynamicCodeGenerated() mutates _runtime_stubs (add()/expand()) concurrently.
+    // Heap usage of the runtime-stubs code cache, via CodeCache::memoryUsage().
+    // Read under the shared stubs lock because DynamicCodeGenerated() mutates
+    // _runtime_stubs (add()/expand()) concurrently.
     static long long runtimeStubsMemoryUsage();
     static bool isJitCode(const void* pc) {
         return CodeHeap::contains(pc);