Add paragraph to lambda chapter explaining tail calls

book.tex

@@ -6331,6 +6331,13 @@ $\Rightarrow$
 The top-level function definitions need to be updated as well to take
 an extra closure parameter.
 
+A final concern when implementing closure conversion is that we want
+to maintain efficient tail calls. To preserve the invariant needed for
+tail calls, \code{limit-functions} should be updated to handle
+\code{lambda} (as it happens before \code{convert-to-closures}), as
+well as to reserve an extra spot for the eventual closure parameter
+for all functions.
+
 \section{An Example Translation}
 \label{sec:example-lambda}