HBASE-23570 Point users to the async-profiler home page if diagrams are coming up blank (#937)

hbase-http/src/main/java/org/apache/hadoop/hbase/http/ProfileServlet.java

@@ -270,7 +270,10 @@ public class ProfileServlet extends HttpServlet {
             resp.getWriter().write(
               "Started [" + event.getInternalName() +
               "] profiling. This page will automatically redirect to " +
-              relativeUrl + " after " + duration + " seconds.\n\nCommand:\n" +
+              relativeUrl + " after " + duration + " seconds. " +
+              "If empty diagram and Linux 4.6+, see 'Basic Usage' section on the Async " +
+              "Profiler Home Page, https://github.com/jvm-profiling-tools/async-profiler." +
+              "\n\nCommand:\n" +
               Joiner.on(" ").join(cmd));
 
             // to avoid auto-refresh by ProfileOutputServlet, refreshDelay can be specified

src/main/asciidoc/_chapters/profiler.adoc

@@ -34,6 +34,9 @@ HBASE-21926 introduced a new servlet that supports integrated profiling via asyn
 == Prerequisites
 
 Go to https://github.com/jvm-profiling-tools/async-profiler, download a release appropriate for your platform, and install on every cluster host.
+If 4.6 or later linux, be sure to set proc variables as per 'Basic Usage' section in the
+<a href="https://github.com/jvm-profiling-tools/async-profiler">Async Profiler Home Page</a>
+(Not doing this will draw you diagrams with no content).
 
 Set `ASYNC_PROFILER_HOME` in the environment (put it in hbase-env.sh) to the root directory of the async-profiler install location, or pass it on the HBase daemon's command line as a system property as `-Dasync.profiler.home=/path/to/async-profiler`.