Fix typo in elpy-shell-add-to-shell-history docstring & enhance flow

Post-edits, this docstring makes it sound like this toggle enables a
kill-ring or clipboard history for the Python shell, which sounds like
it's always a good thing.  That said, it's disabled by default.  I
believe it would be useful to let the user know why it's disabled by
default.

Also, I think it would be useful to have a follow up commit that
explains how it "affects the following functions:…".

elpy-shell.el

@@ -155,10 +155,11 @@ the code cell beginnings defined here."
   :group 'elpy)
 
 (defcustom elpy-shell-add-to-shell-history nil
-  "If Elpy should make the code sent to the shell available in the
-shell history. This allows to use `comint-previous-input' in the
-python shell to get back the pieces of code sent by Elpy. This affects
-the following functions:
+  "Toggle that affects whether Elpy makes the code that is sent
+to the Python shell available in the shell's history.  If
+enabled, use `comint-previous-input' in the Python shell to get
+back the pieces of code sent by Elpy. This affects the following
+functions:
 - `elpy-shell-send-statement'
 - `elpy-shell-send-top-statement'
 - `elpy-shell-send-group'