Consistently use "integrate" as the verb and "relation" as the noun in documentation

[tonyandrewmeyer]

Our API documentation has various places that say "related to" that should be "integrated with" (and so on) so that we are consistent with the main documentation and with Juju. These should be adjusted, keeping "relation" as the noun - ie. we integrate two applications to create a relation.

We could also scan over the other documentation, although @tmihoc is generally handling that for this issue.

We should also discuss whether TooManyRelatedAppsError should be renamed TooManyIntegratedAppsError (with an alias for backwards compatibility).