Prefer "similar" over "equivalent" in tutorial (python#125343)

In the datastructures tutorial doc, some operations are described as
"equivalent to" others. This has led to some user-confusion -- at
least in the Discourse forums -- about cases in which the operations
differ.

This change doesn't systematically eliminate the word "equivalent"
from the tutorial. It just substitutes "similar to" in several cases
in which "equivalent to" could mislead users into expecting exact
equivalence.

Author: sirosen

Doc/tutorial/datastructures.rst

@@ -19,13 +19,13 @@ objects:
 .. method:: list.append(x)
    :noindex:
 
-   Add an item to the end of the list.  Equivalent to ``a[len(a):] = [x]``.
+   Add an item to the end of the list.  Similar to ``a[len(a):] = [x]``.
 
 
 .. method:: list.extend(iterable)
    :noindex:
 
-   Extend the list by appending all the items from the iterable.  Equivalent to
+   Extend the list by appending all the items from the iterable.  Similar to
    ``a[len(a):] = iterable``.
 
 
@@ -56,7 +56,7 @@ objects:
 .. method:: list.clear()
    :noindex:
 
-   Remove all items from the list.  Equivalent to ``del a[:]``.
+   Remove all items from the list.  Similar to ``del a[:]``.
 
 
 .. method:: list.index(x[, start[, end]])
@@ -93,7 +93,7 @@ objects:
 .. method:: list.copy()
    :noindex:
 
-   Return a shallow copy of the list.  Equivalent to ``a[:]``.
+   Return a shallow copy of the list.  Similar to ``a[:]``.
 
 
 An example that uses most of the list methods::