openat2.2: Reorganize and rework introductory text a little

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

man2/openat2.2

@@ -40,6 +40,12 @@ There is no glibc wrapper for this system call; see NOTES.
 .SH DESCRIPTION
 The
 .BR openat2 ()
+system call is an extension of
+.BR openat (2)
+and provides a superset of its functionality.
+.PP
+The
+.BR openat2 ()
 system call opens the file specified by
 .IR pathname .
 If the specified file does not exist, it may optionally (if
@@ -72,11 +78,6 @@ in which case
 is resolved relative to
 .IR dirfd ).
 .PP
-The
-.BR openat2 ()
-system call is an extension of
-.BR openat (2)
-and provides a superset of its functionality.
 Rather than taking a single
 .I flags
 argument, an extensible structure (\fIhow\fP) is passed to allow for
@@ -87,12 +88,15 @@ argument must be specified as
 .IR "sizeof(struct open_how)" .
 .\"
 .SS The open_how structure
-The following structure indicates how
+The
+.I how
+argument specifies how
 .I pathname
 should be opened, and acts as a superset of the
 .IR flags " and " mode
 arguments to
 .BR openat (2).
+This argument is a pointer to a structure of the following form:
 .PP
 .in +4n
 .EX