DOCSP-30862 Document excluded-by-default databases and collections (#131)

* DOCSP-30862 Document excluded-by-default databases and collections

* Apply suggestions from code review

Co-authored-by: Dave Rolsky <autarch@urth.org>

* DOCSP-30862 updates for feedback

* Apply suggestions from code review

Co-authored-by: Nick Villahermosa <nick.villahermosa@mongodb.com>

* DOCSP-30862 updates for NV's feedback

---------

Co-authored-by: Dave Rolsky <autarch@urth.org>
Co-authored-by: Nick Villahermosa <nick.villahermosa@mongodb.com>

Author: 3 people

source/mongodump.txt

@@ -302,8 +302,45 @@ value overrides the read preference specified in the URI string.
 Data Exclusion
 ~~~~~~~~~~~~~~
 
-:binary:`~bin.mongodump` excludes the content of the ``local`` database
-in its output.
+If you specify both the :option:`--db <mongodump --db>` and the
+:option:`--collection <mongodump --collection>` options, ``mongodump``
+dumps the specified collection.
+
+If you only specify the :option:`--db <mongodump --db>` option,
+``mongodump`` dumps that database with the following exclusions:
+
+- If the value of ``--db`` is ``admin``:
+
+  - If you aren't connected to an Atlas Proxy, ``mongodump`` dumps the
+    database but excludes the ``system.keys`` collection. 
+  - If you're connected to an Atlas Proxy, ``mongodump`` attempts to
+    dump the database but errors out.
+
+- If the value of ``--db`` is ``config``, ``mongodump`` dumps all
+  collections in that database. 
+
+- If the value of ``--db`` is ``local``, ``mongodump`` dumps all
+  collections in that database.  
+  
+- For all other databases, ``mongodump`` excludes all collections that
+  start with ``system.``, except for ``system.js``. 
+
+If you omit the ``--db`` option, the following exclusions apply: 
+
+- ``mongodump`` excludes the ``local`` database.
+- ``mongodump`` only includes the following collections in the output
+  for the ``config`` database:   
+
+  - ``chunks``
+  - ``collections``
+  - ``databases``
+  - ``settings``
+  - ``shards``
+  - ``tags``
+  - ``version``
+    
+- If you're connected to an Atlas Proxy, ``mongodump`` excludes the
+  ``admin`` database. 
 
 :binary:`~bin.mongodump` output: