bpo-37134: Add PEP570 notation to the documentation (GH-13743)

diff --git a/Doc/library/struct.rst b/Doc/library/struct.rst
index 1a0fd73..a06d903 100644
--- a/Doc/library/struct.rst
+++ b/Doc/library/struct.rst
@@ -70,7 +70,7 @@
    size required by the format, as reflected by :func:`calcsize`.
 
 
-.. function:: unpack_from(format, buffer, offset=0)
+.. function:: unpack_from(format, /, buffer, offset=0)
 
    Unpack from *buffer* starting at position *offset*, according to the format
    string *format*.  The result is a tuple even if it contains exactly one