Docment the acceptable values for the id parameter.

lib/libc/sys/quotactl.2

@@ -54,7 +54,7 @@ given by
 .Fa cmd
 operates on the given filename
 .Fa path
-for the given user
+for the given user or group
 .Fa id .
 (NOTE: One should use the QCMD macro defined in
 .In ufs/ufs/quota.h
@@ -65,6 +65,12 @@ The address of an optional command specific data structure,
 may be given; its interpretation
 is discussed below with each command.
 .Pp
+For commands that use the
+.Fa id
+identifier, it must be either -1 or any positive value.
+The value of -1 indicates that the current UID or GID should be used.
+Any other negative value will return an error.
+.Pp
 Currently quotas are supported only for the
 .Dq ufs
 file system.
@@ -175,6 +181,15 @@ In
 and
 .Dv Q_SETQUOTA ,
 quotas are not currently enabled for this file system.
+.Pp
+The
+.Fa id
+argument to
+.Dv Q_GETQUOTA ,
+.Dv Q_SETQUOTA 
+or
+.Dv Q_SETUSE
+is a negative value.
 .It Bq Er EACCES
 In
 .Dv Q_QUOTAON ,