Clarify "A note about units" on standard name table

[rljacob]

Many people are confused about if canonical units are required. The helpful "A note about units" on the standard name table (http://cfconventions.org/Data/cf-standard-names/current/build/cf-standard-name-table.html) tries to clarify but buries the useful example about temperature at the end and so many people don't see it. Suggested rewording:

"A note about units
As an example, a variable with the standard name of "air_temperature" may have a units attribute of "degree_Celsius" even though "degree_Kelvin" is the cannonical unit because Celsius can be converted to Kelvin by Udunits. For the full range of supported units, refer to section 6 of the Udunits documentation. Refer to the CF conventions for full details of the units attribute.

The canonical units associated with each standard name are usually the SI units for the quantity. Section 3.3 of the CF conventions states: "Unless it is dimensionless, a variable with a standard_name attribute must have units which are physically equivalent (not necessarily identical) to the canonical units, possibly modified by an operation specified by either the standard name modifier ... or by the cell_methods attribute." Furthermore, Section 1.3 of the CF conventions states: "The values of the units attributes are character strings that are recognized by UNIDATA's Udunits package [UDUNITS], (with exceptions allowed as discussed in Section 3.1, “Units”)."

[rljacob]

Related issue cf-convention/vocabularies#125

[larsbarring]

This would be relevant to revisit once cf-convention/cf-conventions#481 is closed