PR #564 finetuning of back-end "availability" terminology

[soxofaan]

This is an auxiliary PR for #564 . To be merged or cherry-picked from

Main goal is to eliminate/reduce "permanent"/"temporary" terminology which might make cause a mismatch in expectations as I tried to argue in #564 (comment)

contents:

• explicitly mention properties federation:backends and federation:missing in the descriptions
• eliminate the word "permanently" (used once for federation:backends), which was redundant in my opinion
• put list of resources in bold to (hopefully) make it more clear that federation:backends is for individual resources and federation:missing is for compiled listings
• reduce usage of "temporary" and replace it with a neutral "not available". I kept it when used in combination "temporarily or unintentionally" where it has value.
• small typo fix of "the a combination"

[m-mohr]

Thanks, that sounds good.