Include/unicodeobject.h contains followed names which are declared as public but are not documented in Doc/c-api/unicode.rst.

PyUnicodeIter_Type
Py_UNICODE_REPLACEMENT_CHARACTER
PyUnicode_Resize
PyUnicode_InternImmortal
PyUnicode_CHECK_INTERNED
PyUnicode_FromOrdinal
PyUnicode_GetDefaultEncoding
PyUnicode_AsDecodedObject
PyUnicode_AsDecodedUnicode
PyUnicode_AsEncodedObject
PyUnicode_AsEncodedUnicode
PyUnicode_BuildEncodingMap
PyUnicode_DecodeCodePageStateful
PyUnicode_Append
PyUnicode_AppendAndDel
PyUnicode_Partition
PyUnicode_RPartition
PyUnicode_RSplit
PyUnicode_IsIdentifier
_PyUnicode_FromId
_PyUnicode_ClearStaticStrings

They should be either documented or declared as private.

PyUnicode_AsEncodedObject only mentioned in Doc/whatsnew/3.3.rst as recomended replacement of PyUnicode_Encode.