getauxval.3: Document ENOENT error

And add an entry to BUGS explaining the ambiguity that was
present before the addition of this error.

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

man3/getauxval.3

@@ -179,7 +179,12 @@ If
 .I type
 is not found, 0 is returned.
 .SH ERRORS
-No errors are diagnosed.
+.TP
+.BR ENOENT " (since glibc 2.19)"
+.\" commit b9ab448f980e296eac21ac65f53783967cc6037b
+No entry corresponding to
+.IR type
+could be found in the auxiliary vector.
 .SH VERSIONS
 The
 .BR getauxval ()
@@ -215,6 +220,15 @@ be obtained via
 see
 .BR proc (5)
 for more information.
+.SH BUGS
+Before the addition of the
+.B ENOENT
+error in glibc 2.19,
+there was no way to unambiguously distinguish the case where
+.I type
+could not be found from the case where the value corresponding to
+.I type
+was zer0.
 .SH SEE ALSO
 .BR secure_getenv (3),
 .BR vdso (7),