Make PrintfFormatter public (#335, #360)

doc/api.rst

@@ -125,12 +125,15 @@ custom argument formatter class::
 .. doxygenclass:: fmt::ArgFormatter
    :members:
 
-Printf formatting functions
----------------------------
+Printf formatting
+-----------------
 
+The header ``fmt/printf.h`` provides ``printf``-like formatting functionality.
 The following functions use `printf format string syntax
 <http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html>`_ with
-a POSIX extension for positional arguments.
+the POSIX extension for positional arguments. Unlike their standard
+counterparts, the ``fmt`` functions are type-safe and throw an exception if an
+argument type doesn't match its format specification.
 
 .. doxygenfunction:: printf(CStringRef, ArgList)
 
@@ -138,6 +141,9 @@ a POSIX extension for positional arguments.
 
 .. doxygenfunction:: sprintf(CStringRef, ArgList)
 
+.. doxygenclass:: fmt::PrintfFormatter
+   :members:
+
 Write API
 =========