Update perf docs a bit

Author: vchuravy

doc/src/manual/environment-variables.md

@@ -313,6 +313,8 @@ event listener for just-in-time (JIT) profiling.
       (`USE_INTEL_JITEVENTS` set to `1` in the build configuration), or
     * [OProfile](http://oprofile.sourceforge.net/news/) (`USE_OPROFILE_JITEVENTS` set to `1`
       in the build configuration).
+    * [Perf](https://perf.wiki.kernel.org) (`USE_PERF_JITEVENTS` set to `1`
+      in the build configuration). This integration is enabled by default.
 
 ### `ENABLE_GDBLISTENER`
 

doc/src/manual/profile.md

@@ -342,7 +342,8 @@ Or similary with `perf` :
 
 ```
 $ ENABLE_JITPROFILING=1 perf record -o /tmp/perf.data --call-graph dwarf ./julia /test/fastmath.jl
-$ perf report --call-graph -G
+$ perf inject --jit --input /tmp/perf.data --output /tmp/perf-jit.data
+$ perf report --call-graph -G -i /tmp/perf-jit.data
 ```
 
 There are many more interesting things that you can measure about your program, to get a comprehensive list