Clarify the maximum value of room power levels

**Link to problem area**:
https://spec.matrix.org/v1.2/client-server-api/#mroompower_levels

**Issue**
The spec doesn't specify the maximum possible power level. The room creator power level is specified as 100, but the documentation doesn't say if a higher value is allowed (i.e. set by automation to disallow certain actions even for room creator/admin).

This is used by https://github.com/digitalentity/matrix_encryption_disabler and https://github.com/fhirfactory/pegacorn-communicate-roomserver/pull/7 to disable the room encryption for certain use-cases, but since it's not documented in the spec it might be considered unsafe.

**Fix**
Document the valid rande for room power level. The reasonable could be i.e. `[0; 255]` to match the `uint8` type (which would also fit into a single byte for bandwidth-efficient binary encoding and transport protocols if necessary).

Open the linked M_ROOM_POWER_LEVELS section in the Matrix Client-Server API specification and compare its stated creator level with the cited implementations and pull request. Resolve and document the valid range for room power levels; done when the specification explicitly states the allowed maximum.

Written by the indexing model from the issue text.