acpi: improvements for Documentation/thermal/sysfs-api.txt

Here's three patches for your consideration:
1/3: reformat for improved readability
2/3: document passive attribute for thermal zones
3/3: move to acpi directory

The first patch mainly reduces the width of the text to make it fit better in normal windows, but also has a number of consistency improvements.

The second patch is the only one that really changes content. It documents an attribute that is currently missing in the document.
Note that I've not checked whether the description of the API still matches the driver code.

The third patch moves the document to an IMO more logical location and removes the thermal directory from the top Documentation level.

Cheers,
FJP