doc: Rename "Portability" to "Thread-safety"

Fixes #545

doc/apiref.rst

@@ -933,8 +933,7 @@ inserted to the object.
     :func:`json_object()`, either explicit or implicit. If this
     function is not called by the user, the first call to
     :func:`json_object()` (either explicit or implicit) seeds the hash
-    function. See :ref:`portability-thread-safety` for notes on thread
-    safety.
+    function. See :ref:`thread-safety` for notes on thread safety.
 
     If repeatable results are required, for e.g. unit tests, the hash
     function can be "unrandomized" by calling :func:`json_object_seed`

doc/index.rst

@@ -41,7 +41,7 @@ Contents
    upgrading
    tutorial
    conformance
-   portability
+   threadsafety
    apiref
    changes
 

doc/threadsafety.rst

@@ -1,11 +1,8 @@
-***********
-Portability
-***********
-
-.. _portability-thread-safety:
-
+*************
 Thread safety
--------------
+*************
+
+.. _thread-safety:
 
 Jansson as a library is thread safe and has no mutable global state.
 The only exceptions are the hash function seed and memory allocation