Update rs.add.txt

Author: milkie

source/reference/method/rs.add.txt

@@ -24,7 +24,7 @@ Definition
 Behavior
 --------
 
-:method:`rs.add()` can, in some cases, force an election for primary
+:method:`rs.add()` can, in some cases, trigger an election for primary
 which will disconnect the shell. In such cases, the :program:`mongo`
 shell displays an error even if the operation succeeds.
 
@@ -56,13 +56,13 @@ Pass a Member Configuration Document to ``rs.add()``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 .. versionchanged:: 3.0.0
-   Previous version required an ``_id`` file in the document you
+   Previous versions required an ``_id`` field in the document you
    passed to :method:`rs.add()`. After 3.0.0 you can omit the
    ``_id`` field in this document.
 
 .. BEHAVIORAL NOTE:
 
-   If you specify the ``_id`` value must be unique. Use
+   If you specify the ``_id`` value, it must be unique. Use
    :method:`rs.conf()` to see the existing ``_id`` values in the
    replica set configuration document, and set ``_id`` to the next
    unused ``_id`` value in the replica set.