Document what installation mode is in inline docs

[ericlewis]

Boone had a good description of installation mode in r34828.

Let's bring this in somewhere useful in inline documentation, as its functionality is not immediately clear without some back story.

[ocean90]

@ericlewis Could you explain what you want to see here? Is it about improving the docs of wp_installing()?

[DrewAPicture]

I think the description in the wp_installing() DocBlock provides adequate context for what the purpose of checking it is.