Issue #14783: Improve int() docstring and also str(), range(), and slice().

This commit rewrites the docstring for int() to incorporate the documentation
changes made in issue #16036.  It also switches the docstrings for int(),
str(), range(), and slice() to use multi-line signatures.
diff --git a/Objects/sliceobject.c b/Objects/sliceobject.c
index d7b97c9..0f4b647 100644
--- a/Objects/sliceobject.c
+++ b/Objects/sliceobject.c
@@ -221,7 +221,8 @@
 }
 
 PyDoc_STRVAR(slice_doc,
-"slice([start,] stop[, step])\n\
+"slice(stop)\n\
+slice(start, stop[, step])\n\
 \n\
 Create a slice object.  This is used for extended slicing (e.g. a[0:10:2]).");