Add rabbit.vm_memory_calculation_strategy to config example

Author: michaelklishin

docs/rabbitmq.config.example

@@ -251,6 +251,11 @@
    %%
    %% {vm_memory_high_watermark_paging_ratio, 0.5},
 
+   %% Selects Erlang VM memory consumption calculation strategy. Can be `rss` or `erlang`,
+   %% `rss` is the default. Introduced in 3.6.11.
+   %% See https://github.com/rabbitmq/rabbitmq-server/issues/1223 for background.
+   %% {vm_memory_calculation_strategy, rss},
+   
    %% Interval (in milliseconds) at which we perform the check of the memory
    %% levels against the watermarks.
    %%