Add comment about where to connect

[justanyone]

All these tutorials should be self-contained and presume no info in previous tutorials.
This means that everywhere we say, "you must update this...", you need to add a
previous statement for, 'connect to this component'.

Many of customers have sharded replicasets. Saying 'first connect, then...' is too vague.
Don't presume the user knows which to connect to - primary, replica, arbiter, all 3, config
server, or mongos. Especially n00bs, don't know which thing to connect to.

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This change is 

[p-mongo]

I believe existing language already covers the proposal made in this PR.

The "procedure" section starts with:

In the mongo shell, use rs.conf() to retrieve the replica set configuration and assign it to a variable. For example:

The rs.conf is linked to the shell helper documentation, which is linked to the server command. This server command as I understand it can be run on any replica set node.

The procedure continues with rs.reconfig which links to its server command documentation. This command states the requirement to run it on the primary:

You must run this command on the admin database of the primary replica set member.

The language suggested in this PR is actually incorrect - reconfiguration must be run on the primary, not on "any replica set member".