plan9.ini(8), boot(8): clarify the use of $rootdir and $rootspec

sys/man/8/boot

@@ -127,15 +127,23 @@ password:
 .PP
 Once connected,
 .I boot
-mounts
-the root file system before
-.B /
+mounts the root file system at
+.B /root
+(with the optional mount spec argument from
+.BR $rootspec )
 and makes the connection available as
 .B #s/boot
 for subsequent processes to
 .B mount
 (see
 .IR bind (2)).
+It then binds
+.B /root
+(or the value specified by
+.BR $rootdir )
+after the
+.BR /
+directory.
 .I Boot
 completes by 
 .IR exec (2)'ing

sys/man/8/plan9.ini

@@ -691,10 +691,24 @@ See
 for more.
 .SS \fLnobootprompt=\fIvalue\fP
 Suppress the
-.L "root from"
+.L bootargs
 prompt and use
 .I value
 as the answer instead.
+.SS \fLrootdir=\fB/root/\fIdir\fP
+.SS \fLrootspec=\fIspec\fP
+Changes the mount arguments for the root file server
+that was specified by
+.I bootargs
+above.
+By changing
+.I dir
+in
+.BR $rootdir ,
+a different sub-directory on the root file server
+can be used as the system root. see
+.IR boot (8)
+for details.
 .SS \fLuser=\fIvalue\fP
 Suppress the
 .L "user"