Explain feature requirements in systemenc doc

doc/tagbar.txt

@@ -496,6 +496,8 @@ system is different from the one set in Vim, i.e. the 'encoding' option. For
 example, if you use a Simplified Chinese Windows version that has a system
 encoding of "cp936", and you have set 'encoding' to "utf-8", then you would
 have to set this variable to "cp936".
+Note that this requires Vim to be compiled with the |+multi_byte| and (unless
+your encoding is latin1) |+iconv| features to work.
 
 Example:
 >