Return the special docstring substitutions.

{{class}} is replaced by the name of the class.

{{class_lower}} is replaced by the name of the class convert to all lower case.

{{package}} is replaced by the name of the package, as defined by the first N . (dot) separated fields of the class’s __module__ attribute, where is N determined by _docstring_package_depth.

Added in version (cfdm): 1.8.7.0

Returns:

tuple

The special docstring substitution identifiers.