doc: interpret-trailers: rewrite new-trailers paragraphs

doc: interpret-trailers: rewrite new-trailers paragraphs

Two commits ago we moved new-trailers paragraph next to each other.
But there is something curious about two of them:

    By default the new trailer will appear at the end of the trailer
    block. [...]

Then a source block and a paragraph later:

    By default, a `<key>=<value>` or `<key>:<value>` argument given
    using `--trailer` will be appended after the existing trailers only
    if [...]

Why are there two paragraphs that talk about how “By default” a trailer
will be appended?

We can make these paragraphs flow better, and with a more distinct
character each, by dividing the flow like this:

1. Declare that we are about to talk about `--trailer` appending
2. Explain the default behavior
3. Explain how this affects the trailer block
4. Then discuss what each trailer line will look like

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>