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.swagger.json

@@ -5048,7 +5048,7 @@
         },
         "private": {
           "type": "boolean",
-          "description": "Whether this invoice should include routing hints for private channels."
+          "description": "Whether this invoice should include routing hints for private channels.\nNote: When enabled, if value and value_msat are zero, a large number of\nhints with these channels can be included, which might not be desirable."
         },
         "add_index": {
           "type": "string",