[docs] [issue23088] Document that PyUnicode_AsUTF8() returns a null-terminated string

[Roundup Robot]

Roundup Robot added the comment:

New changeset 99d2f83290c0 by R David Murray in branch '3.4':
#23088: Clarify null termination of bytes and strings in C API.
https://hg.python.org/cpython/rev/99d2f83290c0

New changeset 863f7c57081b by R David Murray in branch 'default':
Merge: #23088: Clarify null termination of bytes and strings in C API.
https://hg.python.org/cpython/rev/863f7c57081b

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nosy: +python-dev

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