Misc improvements to the appservice API layout

[turt2live]

This is more of a list of things to keep in mind for when the spec is nearly ready for a release.

• Move the APIs to sections where it makes more sense (ie: /transaction should probably be under "Pushing Events") - AppServices: We need to specify the API endpoints under Querying #1325
• s/AS/application service/ and s/HS/homeserver

[turt2live]

This happened.