Clean up some markup cruft. A number of the macros that take no

parameters (like \UNIX) are commonly entered using an empty group to
separate the markup from a following inter-word space; this is not
needed when the next character is punctuation, or the markup is the
last thing in the enclosing group.  These cases were marked
inconsistently; the empty group is now *only* used when needed.

Doc/api/intro.tex

@@ -3,7 +3,7 @@
 
 The Application Programmer's Interface to Python gives C and
 \Cpp{} programmers access to the Python interpreter at a variety of
-levels.  The API is equally usable from \Cpp{}, but for brevity it is
+levels.  The API is equally usable from \Cpp, but for brevity it is
 generally referred to as the Python/C API.  There are two
 fundamentally different reasons for using the Python/C API.  The first
 reason is to write \emph{extension modules} for specific purposes;