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/unicodeobject.c b/Objects/unicodeobject.c
index f59db36..1dd3a85 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -9986,7 +9986,8 @@
 }
 
 PyDoc_STRVAR(unicode_doc,
-             "str(object[, encoding[, errors]]) -> str\n\
+"str(object='') -> str\n\
+str(bytes_or_buffer[, encoding[, errors]]) -> str\n\
 \n\
 Create a new string object from the given object. If encoding or\n\
 errors is specified, then the object must expose a data buffer\n\