Make docs stricter on variable naming

[BSchilperoort]

currently the docs mostly state the following on naming:

The names are preferably in the form of CSDMS Standard Names.

However, this is a "preference". In some systems using BMI (for example remotebmi) things will completely break if users use characters which are not URL safe.

I would prefer it if the wording on (variable) naming was stronger; for example limiting it to alpha-numeric characters and underscores (A-z, 0-9, and _ ).

[PhilMiller]

It might be preferable, but regardless, I think remotebmi and the like probably needs to escape any strings coming from model code, rather than passing them through verbatim.