doc: add note to folderspec section that MUA root is not MAPI root

References: DESK-3426

Author: jengelh

doc/gromox-mbop.8

@@ -405,5 +405,8 @@ Using a symbolic name: SENT/2022
 .IP \(bu 4
 Referencing a folder with a slash can be done by using backslash as the
 hierarchy separator: SENT\\Winter break 2022/2023
+.PP
+The MAPI root is not visible in most clients. MUAs like Outlook and
+grommunio-web show hierarchy starting at IPM_SUBTREE only.
 .SH See also
 \fBgromox\fP(7)