net/ieee802154_security doc: Shape security expectations

[chrysn]

Contribution description

The security module introduced in #15150 was, in my understanding, not intended for security critical use ("waives to use a proper key store to mitigate complexity"), but that didn't come across clearly in the documentation, and might thus give wrong impressions.

Testing procedure

• Doc only change: Look at the new infobox in ieee802154__security_8h.html group__net__ieee802154__security.html of the built documentation.

Issues/PRs references

#15150 was where the module was introduced

[edit: Hotfixed after I found that the box wound up in the header file and not the module description]

[chrysn]

Deep CI link to the changed spot in the documentation