BatteryManager: Consider merging "multiple batteries" section into original attribute/internal slot definitions

[rakuco]

This originally came up in #46. Rather than having a separate section about what should be done when more than one battery is present, the contents there should be merged into the section where we define how things work when there is only one battery available.

In @marcoscaceres' words:

As a followup, I wonder if this should be actually integrated into the definition of each slot. That way, it's all kept together.

Single battery, do x...

Multiple batteries, do y...