Acquire time is in milliseconds

[ronpandolfi]

The acquire time value is in units of milliseconds, however the EGU field is unpopulated. I haven't found any documentation describing this, so the behavior was unexpected until we discovered this. One or more of the following would help:

• Change to units to seconds and convert internally (personally preferred)
• Add to the documentation that the units for this pv are in ms
• Populate the EGU field to indicate this

I don't know if there are explicit units conventions for areadetector, but I do commonly see seconds for this pv among other device classes. As this is a standard pv, it might be good to sync with others?