doc: adds more clarity on rss

A little more clarity on `rss` to help people disambiguate it from
the virtual memory size.

Refs: #16566 (comment)

doc/api/process.md

@@ -1282,8 +1282,9 @@ Will generate:
 
 `heapTotal` and `heapUsed` refer to V8's memory usage.
 `external` refers to the memory usage of C++ objects bound to JavaScript
-objects managed by V8. `rss`, Resident Set Size, is the total space allocated in
-memory for the process, which includes the _heap_, _code segment_ and _stack_.
+objects managed by V8. `rss`, Resident Set Size, is the amount of space
+occupied in the main memory device (that is a subset of the total allocated
+memory) for the process, which includes the _heap_, _code segment_ and _stack_.
 
 The _heap_ is where objects, strings and closures are stored. Variables are
 stored in the _stack_ and the actual JavaScript code resides in the