The docstring is more accurate.
">>> str.translate.__doc__
'S.translate(table) -> str\n\nReturn a copy of the string S, where all characters have been mapped\nthrough the given translation table, which must be a mapping of\nUnicode ordinals to Unicode ordinals, strings, or None.\nUnmapped characters are left untouched. Characters mapped to None\nare deleted.'""

To me, even this is a bit unclear on exceptions and 'unmapped'. Based on experiments and then reading the C source, I determined that LookupErrors mean 'unmapped' while other exceptions are passed on and terminate the translation.

"Return a copy of the string S, where all characters have been mapped through the given translation table. When subscripted by a Unicode ordinal (integer in range(1048576)), the table must return a Unicode ordinal, string, or None, or else raise a LookupError. A LookupError, which includes instances of subclasses IndexError and KeyError, indicates that the character is unmapped and should be left untouched. Characters mapped to None are deleted."

class Table:
    def __getitem__(self, key):
        if key == 99:   raise LookupError() #'c'
        elif key == 100: return None  # 'd'
        elif key == 101: return 'xyz'  # 'e'
        else: return key+1
                
print('abcdef'.translate(Table()))
# bccxyzg

The current doc ends with "Note
An even more flexible approach is to create a custom character mapping codec using the codecs module (see encodings.cp1251 for an example)."

I don't see how this is supposed to help. Encodings.cp1251 uses a string of 256 chars as a lookup table.