update the documentation about the new strings/seqs behaviours

doc/tut1.rst

@@ -960,11 +960,7 @@ enforced. For example, when reading strings from binary files, they are merely
 a sequence of bytes. The index operation ``s[i]`` means the i-th *char* of
 ``s``, not the i-th *unichar*.
 
-String variables are initialized with a special value, called ``nil``. However,
-most string operations cannot deal with ``nil`` (leading to an exception being
-raised) for performance reasons. It is best to use empty strings ``""``
-rather than ``nil`` as the *empty* value. But ``""`` often creates a string
-object on the heap, so there is a trade-off to be made here.
+String variables are initialized with the empty strings ``""``.
 
 
 Integers
@@ -1309,11 +1305,7 @@ Example:
     x: seq[int] # a reference to a sequence of integers
   x = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence allocated on the heap
 
-Sequence variables are initialized with ``nil``. However, most sequence
-operations cannot deal with ``nil`` (leading to an exception being
-raised) for performance reasons. Thus one should use empty sequences ``@[]``
-rather than ``nil`` as the *empty* value. But ``@[]`` creates a sequence
-object on the heap, so there is a trade-off to be made here.
+Sequence variables are initialized with ``@[]``.
 
 The ``for`` statement can be used with one or two variables when used with a
 sequence. When you use the one variable form, the variable will hold the value
@@ -1355,11 +1347,9 @@ type does not matter.
 .. code-block:: nim
     :test: "nim c $1"
   var
-    fruits:   seq[string]       # reference to a sequence of strings that is initialized with 'nil'
+    fruits:   seq[string]       # reference to a sequence of strings that is initialized with '@[]'
     capitals: array[3, string]  # array of strings with a fixed size
 
-  fruits = @[]                  # creates an empty sequence on the heap that will be referenced by 'fruits'
-
   capitals = ["New York", "London", "Berlin"]   # array 'capitals' allows assignment of only three elements
   fruits.add("Banana")          # sequence 'fruits' is dynamically expandable during runtime
   fruits.add("Mango")
@@ -1691,7 +1681,7 @@ rules apply:
   write(stdout, x(3))   # no error: A.x is called
   write(stdout, x(""))  # no error: B.x is called
 
-  proc x*(a: int): string = nil
+  proc x*(a: int): string = discard
   write(stdout, x(3))   # ambiguous: which `x` is to call?