Merge branch 'sverker/erts/system_monitor-doc/ERL-936' into maint

* sverker/erts/system_monitor-doc:
  erts: Add some erl_nif doc clarifications
  erts: Clarify erlang:system_monitor/2 semantics

2 files changed, 19 insertions, 0 deletions

diff --git a/erts/doc/src/erl_nif.xml b/erts/doc/src/erl_nif.xml
index f88d255296..d74ae23a93 100644
--- a/erts/doc/src/erl_nif.xml
+++ b/erts/doc/src/erl_nif.xml
@@ -1495,6 +1495,9 @@ enif_free_iovec(iovec);]]></code>
           <c>term</c>.</p>
         <p>Returns <c>true</c> on success, or <c>false</c> if <c>term</c> is
           not a handle to a resource object of type <c>type</c>.</p>
+	<p><c>enif_get_resource</c> does not add a reference to the resource
+	  object. However, the pointer received in <c>*objp</c> is guaranteed to
+	  be valid at least as long as the resource handle <c>term</c> is valid.</p>
       </desc>
     </func>
 
@@ -2947,6 +2950,10 @@ enif_map_iterator_destroy(env, &amp;iter);</code>
           References made by <seealso marker="#enif_make_resource">
           <c>enif_make_resource</c></seealso>
           can only be removed by the garbage collector.</p>
+	<p>There are no guarantees exactly when the destructor of an
+	  unreferenced resource is called. It could be called directly by
+	  <c>enif_release_resource</c> but it could also be scheduled to be
+	  called at a later time possibly by another thread.</p>
       </desc>
     </func>
 
diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml
index 68cde567c2..f41ed87048 100644
--- a/erts/doc/src/erlang.xml
+++ b/erts/doc/src/erlang.xml
@@ -9413,6 +9413,18 @@ Metadata = #{ pid => pid(),
         <p>Returns the previous system monitor settings just like
           <seealso marker="#system_monitor/0">
           <c>erlang:system_monitor/0</c></seealso>.</p>
+	<p>The arguments to <c>system_monitor/2</c> specifies how all
+	  system monitoring on the node should be done, not how it should be
+	  changed. This means only one process at a time
+	  (<c><anno>MonitorPid</anno></c>) can be the receiver of system monitor
+	  messages. Also, the way to clear a specific monitor option
+	  is to not include it in the list <c><anno>Options</anno></c>. All
+	  system monitoring will, however, be cleared if the process identified by
+	  <c><anno>MonitorPid</anno></c> terminates.</p>
+	<p>There are no special option values (like zero) to clear an option.
+	  Some of the options have a unspecified minimum value. Lower values
+	  will be adjusted to the minimum value. For example, it is currently not
+	  possible to monitor all garbage collections with <c>{long_gc, 0}</c>.</p>
         <note>
           <p>If a monitoring process gets so large that it itself
             starts to cause system monitor messages when garbage