(DOCS-9808): mongo --host supports connection strings

[nlarew]

Jira: DOCS-9808

Staged: mongo --host

This update (finally) explicitly lists connection strings as a valid argument for the mongo --host option. The ticket notes a brief functionality regression in 3.4 (fixed in 3.4.2), however I'm not sure that we should list that as the hotfix was included in a minor version bump.

The update also explicitly calls out DNS seedlist connections strings (mongodb+srv://...) so we can remove some of the extraneous information about it. (redundant info with the main seedlist page).

[nlarew]

@rkumar-mongo updated per your review