Provide more helpful box-sizing guidance for third party coexistence

[tardate]

The box-sizing guidance for third party co-existence was made more general on #10278 (previously it was quite specific to Google Maps - see #9175).

However the 'problem' is very common as noted on #10278 (I just discovered two more - the ZenDesk support widget and the Janrain Engage sign-in widget), but the latest guidance in the Third Party box-sizing doc is a bit weak on two points:

• The wording is now so generic it does not help us recognize when the advice may apply. In particular 'Certain third party tools—such as Google Maps—have trouble working out of the box' does not identify the symptoms and so it is not clear what 'working out of the box' means in this context.
• The advice to wrap offending third party code is often not possible because the code is dynamically generated and injected beyond our control (without hacking the internals of the third party product).

As a suggestion to improve the section and make it a bit more helpful, I've reworded the description to be clearer about the symptoms and situations this problem might arise, and added another solution for when the first does not apply.

What you think of this version?

NB1: I tried to avoid mentioning third party products specifically, but I think the example required it.

NB2: Calling this section "Third Party Support" is a bit of a misnomer as it is more about "Third Party Coexistence" but I refrained from conflating the issue. Worth changing perhaps?

[Merg1255]

add: the gmaps example should rather be on the next upcoming version.

[mdo]

We should be good to go on this—I refactored this a bit last night. That said we could probably add the ZenDesk example to the list.