Clarifying the documentation for diagnostic formats; NFC

While discussing diagnostic format strings with a GSoC mentee, it
became clear there was some confusion regarding how to use them.
Specifically, the documentation for %select caused confunsion because
it was using %select{}2 and talking about how the integer value must
be in the range [0..2], which made it seem like the positional argument
was actually specifying the range of acceptable values.

I clarified several of the examples similarly, moved some documentation
to a more appropriate place, and added some additional information to
the %s modifier to point out that %plural exists.