Inconsistent OpenAPI titles

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The OpenAPI titles in data/api/ and data/schemas/ are suitable to use as class names in auto-generated code (after transformations such as remove spaces). They look like this (random selection):
```
Device List Update
KeyObject
PDU Processing Result
Account Data
Field Type
Event Container
Signatures
PaginationChunk
AssociatedMappings
ToDevice
CreationContent
ConnectionInfo
Membership Event Content
Groups
Results
```

Most titles in data/event-schemas/schema/ are similar. These titles are the exceptions:
```
Accepted Terms of Service URLs
An invitation to a room issued to a third party identifier, rather than a matrix user ID.
Controls visibility of history.
Controls whether guest users are allowed to join rooms.
Defines the power levels (privileges) of users in the room.
Describes how users are allowed to join the room.
Indication that the room has been upgraded.
Informs the room as to which alias is the canonical one.
Pinned events in a room
The current membership state of a user in the room.
The first event in the room.
```

Review the OpenAPI title fields in data/api/, data/schemas/, and data/event-schemas/schema/, comparing the listed exceptions with the surrounding titles. Update the inconsistent titles so they are suitable for use as class names after normal transformations, then verify that the affected titles follow the same convention as the others.

Written by the indexing model from the issue text.