lnrpc+cmd: clarify docs about addinvoice no-amount hop hints

Add additional comment to `lnclic addinvoice`'s `--private` flag warning
about including a large number of private channel hints when using zero
ammounts.

Also added a comment to the `private` field in the `Invoice` proto
message.

lnrpc/lightning.proto

@@ -3401,6 +3401,8 @@ message Invoice {
     repeated RouteHint route_hints = 14;
 
     // Whether this invoice should include routing hints for private channels.
+    // Note: When enabled, if value and value_msat are zero, a large number of
+    // hints with these channels can be included, which might not be desirable.
     bool private = 15;
 
     /*