doc: document restrictions on invoice RPC.

In particular: label must be a string, and description cannot use any
the JSON unicode escapes, which should be unnecessary with UTF-8
anyway.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-invoice.7.txt

@@ -19,13 +19,14 @@ lightning daemon can use to pay this invoice.
 The 'msatoshi' can be the string "any", which creates an invoice
 that can be paid with any amount.
 
-The 'label' must be unique; it is never revealed to other nodes on
+The 'label' must be a unique string; it is never revealed to other nodes on
 the lightning network, but it can be used to query the status of this
 invoice.
 
 The 'description' is a short description of purpose of payment,
 e.g. '1 cup of coffee'. This value is encoded into the BOLT11 invoice
-and is viewable by any node you send this invoice to.
+and is viewable by any node you send this invoice to.  It must be
+UTF-8, and cannot use '\u' JSON escape codes.
 
 The 'expiry' is optionally the number of seconds the invoice is valid for.
 If no value is provided the default of 3600 (1 Hour) is used.