I contacted PyCharm support, and this is what they said:
For tuple please use (<type_1>, <type_2>, <type_3>, e t.c.)
syntax.
E.g.:
"""
:rtype: (string, int, int)
"""
This is confirmed in PyCharm's documentation:
Type Syntax
Type syntax in Python docstrings is not defined by any standard. Thus, PyCharm suggests the following notation:
...
- (Foo, Bar) # Tuple of Foo and Bar
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