man: rework the text in sd-id128

man: rework the text in sd-id128

In places the text was overly formal, e.g. "an 128-bit ID" was repeated, even
though it is clear from the context that we're talking about this type of ID.
OTOH, in other places the text was informal, e.g. "You can use …".
Also, "you may use f() to frob" → "f() frobs". The text without all the
flourishes is easier to read.

sd_id128_in_set_sentinel() was described only in passing when taking about
sd_id128_in_set(), now it gets is own brief paragraph.

The synopsis was missing.