Implement documentation for plgcmap1_range and plscmap1_range.
Thanks to Hǎiliàng Wang for writing the plplot.h/api.xml
consistency checker that discovered these functions in our
public API did not have any documentation in api.xml.
ToDo: There are still 4 more functions that are missing documentation
according to that checker report and also inconsistencies between
plplot.h and api.xml in the number and names of the function