[eia] add better names / descriptions

[beets]

https://autopush.datacommons.org/tools/timeline#place=geoId%2F06%2CgeoId%2F26&statsVar=Annual_Average_HeatContent_Coal_For_CommercialAndInstitutional&chart=%7B%22consumption%22%3Afalse%2C%22cost%22%3Afalse%7D

it's unclear from the name that it only applies to coal. we should revisit all the coal variables, at least. and add descriptions where we can. would be best if we could link back to EIA for the definition

cc @pradh

[beets]

here are other examples: https://autopush.datacommons.org/tools/timeline#place=geoId%2F06%2CgeoId%2F26&statsVar=Annual_Consumption_Coal_ElectricPower&chart=%7B%22consumption%22%3Afalse%2C%22cost%22%3Afalse%2C%22heatContent%22%3Afalse%7D https://autopush.datacommons.org/tools/timeline#place=geoId%2F06%2CgeoId%2F26&statsVar=Quarterly_Average_Cost_Coal_ElectricPower&chart=%7B%22consumption%22%3Afalse%7D https://autopush.datacommons.org/tools/timeline#place=geoId%2F06%2CgeoId%2F26&statsVar=Annual_Consumption_Coal_ElectricPower&chart=%7B%22consumption%22%3Afalse%2C%22cost%22%3Afalse%2C%22heatContent%22%3Afalse%7D

[beets]

cc @n-h-diaz

[hareesh-ms]

Tracked in internal bug.