lnrpc: add devrpc doc tags to enable lncli api doc generation

lnrpc/devrpc/dev.proto

@@ -6,8 +6,26 @@ package devrpc;
 
 option go_package = "github.com/lightningnetwork/lnd/lnrpc/devrpc";
 
+/*
+ * Comments in this file will be directly parsed into the API
+ * Documentation as descriptions of the associated method, message, or field.
+ * These descriptions should go right above the definition of the object, and
+ * can be in either block or // comment format.
+ *
+ * An RPC method can be matched to an lncli command by placing a line in the
+ * beginning of the description in exactly the following format:
+ * lncli: `methodname`
+ *
+ * Failure to specify the exact name of the command will cause documentation
+ * generation to fail.
+ *
+ * More information on how exactly the gRPC documentation is generated from
+ * this proto file can be found here:
+ * https://github.com/lightninglabs/lightning-api
+ */
+
 service Dev {
-    /*
+    /* lncli: `importgraph`
     ImportGraph imports a ChannelGraph into the graph database. Should only be
     used for development.
     */