doc: add details about rss on process.memoryUsage

1. `process.memoryUsage()` returns an object with 4 keys: `rss,
heapTotal, headUsed, external`. There were brief explanations for
the rest except `rss`. This commit adds this on the docs.

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

PR-URL: #16566
Refs: #16566 (comment)
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/process.md

@@ -1196,7 +1196,13 @@ 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.
+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
+_code segment_.
 
 ## process.nextTick(callback[, ...args])
 <!-- YAML