Review schema descriptions

There are descriptions in `jans_schema.json` looking off or that can be enhanced at least. Examples:

- Several attributes have `"desc": "jans Policy URI"`.
- Extremely concise like `"desc": "jans Req Obj Enc Alg"` (average dev cannot guess what this is about)

If the schema had better descriptions we would avoid creating duplicate attributes.

Start by reviewing the descriptions in jans_schema.json, especially attributes using "jans Policy URI" and "jans Req Obj Enc Alg". Identify other overly terse or misleading descriptions and compare nearby schema entries for context. Done means the relevant descriptions clearly explain their attributes and reduce the risk of duplicate attributes.

Written by the indexing model from the issue text.