fpathconf.3: Rework RETURN VALUE description to add more detail

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

man3/fpathconf.3

@@ -158,14 +158,43 @@ or
 .I path
 must refer to a terminal.
 .SH RETURN VALUE
-The limit is returned, if one exists.
-If the system does not have a
-limit for the requested resource, \-1 is returned, and
+The return value of these functions is one of the following:
+.IP * 3
+On error, \-1 is returned and
 .I errno
-is unchanged.
-If there is an error, \-1 is returned, and
+is set to indicate the cause of the error
+(for example,
+.BR EINVAL ,
+indicating that
+.I name
+is invalid).
+.IP *
+If
+.I name
+corresponds to a maximum or minimum limit, and that limit is indeterminate,
+\-1 is returned and
 .I errno
-is set to reflect the nature of the error.
+is not changed.
+(To distinguish an indeterminate limit from an error, set
+.I errno
+to zero before the call, and then check whether
+.I errno
+is nonzero when \-1 is returned.)
+.IP *
+If
+.I name
+corresponds to an option,
+a positive value is returned if the option is supported,
+and \-1 is returned if the option is not supported.
+.IP *
+Otherwise,
+the current value of the option or limit is returned.
+This value will not be more restrictive than
+the corresponding value that was described to the application in
+.I <unistd.h>
+or
+.I <limits.h>
+when the application was compiled.
 .SH ATTRIBUTES
 For an explanation of the terms used in this section, see
 .BR attributes (7).