man: reorder fields in os-release

Let's order the fields from the most general to least: os name, os variant, os
version, machine-parseable version details, metadata, special settings. I added
section headers to roughly group the settings. The division is not strict,
because for example CPE_NAME also includes the version, and PRETTY_NAME may
too, but it still makes it easier to find the right name.

Also split out Examples to separate paragraphs:
almost all descriptions had "Example:" at the end, where multiple
examples were listed. Splitting this out to separate paragraphs
makes the whole thing much easier to read.

Add missing markup and punctuation while at it.

About
- If not set, defaults to <literal>NAME=Linux</literal>.
+ If not set, a default of <literal>NAME=Linux</literal> may be used.
and similar changes: in many circumstances, if this is not set, no value should
be used. The fallback mostly make sense when we need to present something to the
user. So let's reword this to not imply that the default is necessary.

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