Add availability information for a couple of the types, and notes on writing

string-type tests for versions of Python built without Unicode support.

Doc/lib/libtypes.tex

@@ -48,7 +48,8 @@ The type of floating point numbers (e.g. \code{1.0}).
 \end{datadesc}
 
 \begin{datadesc}{ComplexType}
-The type of complex numbers (e.g. \code{1.0j}).
+The type of complex numbers (e.g. \code{1.0j}).  This is not defined
+if Python was built without complex number support.
 \end{datadesc}
 
 \begin{datadesc}{StringType}
@@ -56,7 +57,8 @@ The type of character strings (e.g. \code{'Spam'}).
 \end{datadesc}
 
 \begin{datadesc}{UnicodeType}
-The type of Unicode character strings (e.g. \code{u'Spam'}).
+The type of Unicode character strings (e.g. \code{u'Spam'}).  This is
+not defined if Python was built without Unicode support.
 \end{datadesc}
 
 \begin{datadesc}{TupleType}
@@ -157,7 +159,10 @@ The type of buffer objects created by the
 \end{datadesc}
 
 \begin{datadesc}{StringTypes}
-A list containing \var{StringType} and \var{UnicodeType} used to
-facilitate easier checking for any string object, e.g. \code{s in
-types.StringTypes}.
+A sequence containing \code{StringType} and \code{UnicodeType} used to
+facilitate easier checking for any string object.  Using this is more
+portable than using a sequence of the two string types constructed
+elsewhere since it only contains \code{UnicodeType} if it has been
+built in the running version of Python.  For example:
+\code{isinstance(s, types.StringTypes)}.
 \end{datadesc}