Minor fast-import documentation corrections.

Minor fast-import documentation corrections.

Corrected a couple of header markup lines which were shorter than the
actual header, and made the `data` commands two formats into a named
list, which matches how we document the two formats of the `M` command
within a commit.

Also tried to simplify the language about our decimal integer format;
Linus pointed out I was probably being too specific at the cost of
reduced readability.

Signed-off-by: Shawn O. Pearce <redacted>