Server discovery step 3 is incredibly long

**Link to problem area**: https://spec.matrix.org/v1.7/server-server-api/#resolving-server-names

**Issue**
All of the caching details should be moved to the schema definition. Also, we say *twice* in the same step that "schema details are defined later in this section".

Start with the Resolving server names section linked in the issue, focusing on server discovery step 3 and the schema definition later in that section. Move the caching details to the schema definition and remove the duplicated statement about schema details; the section should retain the same meaning without repeating that note.

Written by the indexing model from the issue text.