add description of LLTRACE.

Misc/SpecialBuilds.txt

@@ -135,13 +135,13 @@ Py_DEBUG                                                  introduced in 1.5
 
 This is what is generally meant by "a debug build" of Python.
 
-Py_DEBUG implies Py_REF_DEBUG, Py_TRACE_REFS, and PYMALLOC_DEBUG (if
-WITH_PYMALLOC is enabled).  In addition, C assert()s are enabled (via
-the C way:  by not defining NDEBUG), and some routines do additional
-sanity checks inside "#ifdef Py_DEBUG" blocks.
+Py_DEBUG implies LLTRACE, Py_REF_DEBUG, Py_TRACE_REFS, and
+PYMALLOC_DEBUG (if WITH_PYMALLOC is enabled).  In addition, C
+assert()s are enabled (via the C way: by not defining NDEBUG), and
+some routines do additional sanity checks inside "#ifdef Py_DEBUG"
+blocks.
 ---------------------------------------------------------------------------
-COUNT_ALLOCS                                            introduced in 0.9.9
-                                             partly broken in 2.2 and 2.2.1
+COUNT_ALLOCS introduced in 0.9.9 partly broken in 2.2 and 2.2.1
 
 Each type object grows three new members:
 
@@ -186,3 +186,15 @@ sys.getcounts()
     for which the first allocation of an object of that type occurred
     most recently is at the front of the list.
 ---------------------------------------------------------------------------
+LLTRACE                                    introduced ...?  Long time ago!
+
+Compile in support of Low Level TRACE-ing of the man interpreter loop.
+
+When this preprocessor symbol is defined, before eval_frame
+(eval_code2 before 2.2) executes a frame's code checks its global
+namespace for a variable "__lltrace__".  If such a variable is found,
+mounds of information about what the interpreter is doing are sprayed
+to stdout, such as every opcode and opcode argument and values pushed
+onto and popped off the value stack.
+
+Not useful very often, but very useful when needed.