docs(connections): improve description of connection creation patterns

[vkarpov15]

Fix #14528

Summary

#14528 points out that we have an example which uses fast.js and slow.js as connection file names without clarifying why one connection is "fast" and the other is "slow". Admittedly the fast/slow distinction is a bit of a red herring in this case, so I'm updating the docs to use a different example of separate connections for web vs mobile.

Examples