gnrc/ipv6_auto_subnets: improve documentation about subnet generation and RIO

[benpicco]

Contribution description

This better illustrates how subnet generation works and why we need the Route Information Option in a Router Advertisement to the upstream network for this to properly work.

Testing procedure

Read the documentation generated by CircleCI and see if things make sense.

Issues/PRs references

[chrysn]

SVG files are primarily edited in GUIs, and here even just checked in pregenerated b/c the actual source can't be built in the automation. Requiring whitespace cleanliness would be excessive here IMO.

(Yes it's kind of a bug in the generator, but then it'll be better here when it''s fixed there, and until that semi-manual cleanups don't do noone no good).

[miri64]

SVG files are primarily edited in GUIs, and here even just checked in pregenerated b/c the actual source can't be built in the automation. Requiring whitespace cleanliness would be excessive here IMO. (Yes it's kind of a bug in the generator, but then it'll be better here when it''s fixed there, and until that semi-manual cleanups don't do noone no good).

Ok, fine...