docs/library/usocket: Move socket.error to its own section.

It's too minor a point to start the module description with it.

docs/library/usocket.rst

@@ -9,12 +9,6 @@
 
 This module provides access to the BSD socket interface.
 
-.. admonition:: Difference to CPython
-   :class: attention
-
-   CPython used to have a ``socket.error`` exception which is now deprecated,
-   and is an alias of `OSError`. In MicroPython, use `OSError` directly.
-
 .. admonition:: Difference to CPython
    :class: attention
 
@@ -250,3 +244,13 @@ Methods
    the length of *buf*.
 
    Return value: number of bytes written.
+
+.. exception:: socket.error
+
+   MicroPython does NOT have this exception.
+
+   .. admonition:: Difference to CPython
+        :class: attention
+
+        CPython used to have a ``socket.error`` exception which is now deprecated,
+        and is an alias of `OSError`. In MicroPython, use `OSError` directly.