errno.3: Minor text reorganization

No content changes.

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

man3/errno.3

@@ -42,7 +42,12 @@ header file defines the integer variable
 .IR errno ,
 which is set by system calls and some library functions in the event
 of an error to indicate what went wrong.
-Its value is significant only when the return value of
+The value of
+.I errno
+is never set to zero by any system call or library function.
+The value in
+.I errno
+is significant only when the return value of
 the call indicated an error
 (i.e., \-1 from most system calls;
 \-1 or NULL from most library functions);
@@ -51,10 +56,7 @@ a function that succeeds
 allowed to change
 .IR errno .
 .PP
-Valid error numbers are all nonzero;
-.I errno
-is never set to zero
-by any system call or library function.
+Valid error numbers are all nonzero.
 .PP
 For some system calls and library functions (e.g.,
 .BR getpriority (2)),