syntax consistency for dimensionless vertical coordinate definitions

[taylor13]

I suggest that for readability and for consistency in the syntax of the definitions of dimensionless vertical coordinates, the following changes be made in the CF documentation.

1. Where the coordinate definition involves a construction like "for k<=k_c .... for k>k_c ....", follow each "for" phrase with a colon. [This is currently done inconsistently.]

2. Where the coordinate definition involves a term that is subsequently defined (e.g., the C(k) in the ocean-S coordinate), precede the definition of the term with "where".

These make the definitions easier to read. Note that CF does not mandate storage of the coordinate definitions in the netCDF files, but for CMIP, we store the formulas as a text string attribute attached to the coordinate variable and named "formula".