Update JavaDoc of SystemClock class (#5477)

Co-authored-by: Jack Berg <jberg@newrelic.com>

sdk/common/src/main/java/io/opentelemetry/sdk/common/SystemClock.java

@@ -8,7 +8,10 @@ package io.opentelemetry.sdk.common;
 import io.opentelemetry.sdk.internal.JavaVersionSpecific;
 import javax.annotation.concurrent.ThreadSafe;
 
-/** A {@link Clock} that uses {@link System#currentTimeMillis()} and {@link System#nanoTime()}. */
+/**
+ * A {@link Clock} that uses {@link JavaVersionSpecific#currentTimeNanos()} and {@link
+ * System#nanoTime()}.
+ */
 @ThreadSafe
 final class SystemClock implements Clock {
 
@@ -16,11 +19,7 @@ final class SystemClock implements Clock {
 
   private SystemClock() {}
 
-  /**
-   * Returns a {@code MillisClock}.
-   *
-   * @return a {@code MillisClock}.
-   */
+  /** Returns a {@link SystemClock}. */
   static Clock getInstance() {
     return INSTANCE;
   }