bpo-30249: Improve struct.unpack_from() error messages (GH-6059)

diff --git a/Modules/clinic/_struct.c.h b/Modules/clinic/_struct.c.h
index 6e43215..2ecadfb 100644
--- a/Modules/clinic/_struct.c.h
+++ b/Modules/clinic/_struct.c.h
@@ -79,7 +79,8 @@
 "\n"
 "Values are unpacked according to the format string Struct.format.\n"
 "\n"
-"The buffer\'s size in bytes, minus offset, must be at least Struct.size.\n"
+"The buffer\'s size in bytes, starting at position offset, must be\n"
+"at least Struct.size.\n"
 "\n"
 "See help(struct) for more on format strings.");
 
@@ -302,4 +303,4 @@
 
     return return_value;
 }
-/*[clinic end generated code: output=9119f213a951e4cc input=a9049054013a1b77]*/
+/*[clinic end generated code: output=d79b009652ae0b89 input=a9049054013a1b77]*/