Update spec status

[rakuco]

This was discussed in TPAC 2021:

RESOLUTION: Update Status of the Document to indicate Magnetometer is looking for developer feedback and high value use cases

[anssiko]

@rakuco, please feel free to share any other resources useful to web developers in #59. For example, some information on device support across OSes might help developers interested in this API.

[Zibri]

I enabled the API but I think the magnetometer api should be like the movement (accelerometer/gyro) api.

and it need at least 60 updates per second (like the other APIs) to be useful for a wider range of application aside from finding metals and "north".

[anssiko]

@Zibri thanks for your feedback. I followed up in #59 (comment) (no worries for commenting in this PR first!).