#14804: Remove [] around optional arguments with default values

Mostly just mechanical removal of []. In some rare cases I've pulled the
default value up into the argument list.
diff --git a/Doc/library/struct.rst b/Doc/library/struct.rst
index 4331665..0016fcb 100644
--- a/Doc/library/struct.rst
+++ b/Doc/library/struct.rst
@@ -386,7 +386,7 @@
       (``len(string)`` must equal :attr:`self.size`).
 
 
-   .. method:: unpack_from(buffer[, offset=0])
+   .. method:: unpack_from(buffer, offset=0)
 
       Identical to the :func:`unpack_from` function, using the compiled format.
       (``len(buffer[offset:])`` must be at least :attr:`self.size`).