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/rangeobject.c b/Objects/rangeobject.c
index 935b205..b67b969 100644
--- a/Objects/rangeobject.c
+++ b/Objects/rangeobject.c
@@ -135,7 +135,8 @@
 }
 
 PyDoc_STRVAR(range_doc,
-"range([start,] stop[, step]) -> range object\n\
+"range(stop) -> range object\n\
+range(start, stop[, step]) -> range object\n\
 \n\
 Returns a virtual sequence of numbers from start to stop by step.");