Explain what file descriptors are; this change has been sitting in my tree for a while
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index dc621dd..2fbecfa 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -427,8 +427,15 @@
\subsection{File Descriptor Operations \label{os-fd-ops}}
-These functions operate on I/O streams referred to
-using file descriptors.
+These functions operate on I/O streams referenced using file
+descriptors.
+
+File descriptors are small integers corresponding to a file that has
+been opened by the current process. For example, standard input is
+usually file descriptor 0, standard output is 1, and standard error is
+2. Further files opened by a process will then be assigned 3, 4, 5,
+and so forth. The name ``file descriptor'' is slightly deceptive; on
+{\UNIX} platforms, sockets and pipes are also referenced by file descriptors.
\begin{funcdesc}{close}{fd}