userfaultfd.2: Add an introductory section on the ioctl(2) operations

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/userfaultfd.2

@@ -112,6 +112,22 @@ operations to resolve the page fault.
 Currently, userfaultfd can be used only with anonymous private memory
 mappings.
 .\"
+.SS ioctl(2) operations
+Various
+.BR ioctl (2)
+operations can be performed on a userfaultfd object using calls of the form:
+
+    ioctl(fd, cmd, argp);
+
+In the above,
+.I fd
+is a file descriptor referring to a userfaultfd object,
+.I cmd
+is one of the commands listed below, and
+.I argp
+is a pointer to a data structure that is specific to
+.IR cmd .
+.\"
 .SS Configuration ioctl(2) operations
 The
 .BR ioctl (2)