Explain what ( . c) means to the Emacs Lisp reader

* doc/lispref/objects.texi (Dotted Pair Notation): Explain what
( . c) means to the Lisp reader (bug#24875).

doc/lispref/objects.texi

@@ -1001,6 +1001,13 @@ It looks like this:
 @end example
 @end ifnottex
 
+  As a somewhat peculiar side effect of @code{(a b . c)} and
+@code{(a . (b . c))} being equivalent, for consistency this means
+that if you replace @code{b} here with the empty sequence, then it
+follows that @code{(a . c)} and @code{(a . ( . c))} are equivalent,
+too.  This also means that @code{( .  c)} is equivalent to @code{c},
+but this is seldom used.
+
 @node Association List Type
 @subsubsection Association List Type