some unicode APIs do not document error behavior

[maxbachmann]

Documentation

I came across some APIs in the unicode C-API, which do not document how they behave in the error case:

• PyUnicode_New
• PyUnicode_GetLength
• PyUnicode_WriteChar
• PyUnicode_ReadChar
• PyUnicode_Substring

• PR: GH-95079: document error behaviour for some unicode C APIs #95080

• PR: [3.13] GH-95079: document error behaviour for some unicode C APIs (GH-95080) #124661

• PR: [3.12] GH-95079: document error behaviour for some unicode C APIs (GH-95080) #124662