[Patch #1490989 from Skip Montanaro] Mention debugging builds in the API…

[Patch #1490989 from Skip Montanaro]  Mention debugging builds in the API documentation.  I've changed Skip's patch to point to Misc/SpecialBuilds and fiddled with the markup a bit.

Doc/api/api.tex

@@ -48,11 +48,6 @@ code releases.}
 \input{newtypes}
 
 
-% \chapter{Debugging \label{debugging}}
-%
-% XXX Explain Py_DEBUG, Py_TRACE_REFS, Py_REF_DEBUG.
-
-
 \appendix
 \chapter{Reporting Bugs}
 \input{reportingbugs}

Doc/api/intro.tex

@@ -580,3 +580,59 @@ information about these functions is given in a later chapter.
 Notice that \cfunction{Py_Finalize} does \emph{not} free all memory
 allocated by the Python interpreter, e.g. memory allocated by extension
 modules currently cannot be released.
+
+
+\section{Debugging Builds \label{debugging}}
+
+Python can be built with several macros to enable extra checks of the
+interpreter and extension modules.  These checks tend to add a large
+amount of overhead to the runtime so they are not enabled by default.
+
+A full list of the various types of debugging builds is in the file
+\file{Misc/SpecialBuilds.txt} in the Python source distribution.
+Builds are available that support tracing of reference counts,
+debugging the memory allocator, or low-level profiling of the main
+interpreter loop.  Only the most frequently-used builds will be
+described in the remainder of this section.
+
+Compiling the interpreter with the \csimplemacro{Py_DEBUG} macro
+defined produces what is generally meant by "a debug build" of Python.
+\csimplemacro{Py_DEBUG} is enabled in the Unix build by adding
+\longprogramopt{with-pydebug} to the \file{configure} command.  It is also
+implied by the presence of the not-Python-specific
+\csimplemacro{_DEBUG} macro.  When \csimplemacro{Py_DEBUG} is enabled
+in the Unix build, compiler optimization is disabled.
+
+In addition to the reference count debugging described below, the
+following extra checks are performed:
+
+\begin{itemize}
+      \item Extra checks are added to the object allocator.
+      \item Extra checks are added to the parser and compiler.
+      \item Downcasts from wide types to narrow types are checked for
+            loss of information.
+      \item A number of assertions are added to the dictionary and set
+            implementations.  In addition, the set object acquires a
+            \method{test_c_api} method.
+      \item Sanity checks of the input arguments are added to frame
+            creation. 
+      \item The storage for long ints is initialized with a known
+            invalid pattern to catch reference to uninitialized
+            digits. 
+      \item Low-level tracing and extra exception checking are added
+            to the runtime virtual machine.
+      \item Extra checks are added to the memory arena implementation.
+      \item Extra debugging is added to the thread module.
+\end{itemize}
+
+There may be additional checks not mentioned here.
+
+Defining \csimplemacro{Py_TRACE_REFS} enables reference tracing.  When
+defined, a circular doubly linked list of active objects is maintained
+by adding two extra fields to every \ctype{PyObject}.  Total
+allocations are tracked as well.  Upon exit, all existing references
+are printed.  (In interactive mode this happens after every statement
+run by the interpreter.)  Implied by \csimplemacro{Py_DEBUG}.
+
+Please refer to \file{Misc/SpecialBuilds.txt} in the Python source
+distribution for more detailed information.