mpc: add documentation to server config example files

testdata/mpc_phase1_server_config.json

@@ -1,17 +1,33 @@
 {
     "server": {
+        "_contributors_file_description": "Relative path of contributors file",
         "contributors_file": "../testdata/mpc_contributors.json",
+
+        "_start_time_description": "Time (local) that first contribution is allowed",
         "_start_time_example": "2019-11-05 13:33:13",
         "start_time": "TIME",
+
         "_contribution_interval_description": "Interval time in seconds",
         "contribution_interval": "10.0",
+
+        "_email_description": "Optional email server and credentials",
         "_email_server": "smtp.myorg.com:465",
         "_email_address": "mpc.coordinator@myorg.com",
         "_email_password_file": "file containing password for email account",
+
+
+        "_tls_description": "Key and certificate for the server.",
         "tls_key": "key.pem",
         "tls_certificate": "cert.pem",
+
+        "_port_description": "Port to listen on",
         "port": PORT
     },
+
+
+    "_pot_path_description": "Path (relative) to the external powersoftau tool",
     "pot_path": "../../powersoftau",
+
+    "_num_powers_description": "Determines max degree of circuit. (Power of 2)",
     "num_powers": QAP_DEGREE
 }

testdata/mpc_phase2_server_config.json

@@ -1,5 +1,8 @@
 {
     "server": {
+        "help1": "This section is common over phase1 and phase2. See ",
+        "help2": "mpc_phase1_server.config.json for details.",
+
         "contributors_file": "../testdata/mpc_contributors.json",
         "start_time": "TIME",
         "contribution_interval": "10.0",
@@ -10,5 +13,7 @@
         "tls_certificate": "cert.pem",
         "port": PORT
     },
+
+    "_mpc_tool_description": "Path (relative) to the phase2 mpc tool",
     "mpc_tool": "../build/mpc_tools/mpc_phase2/mpc-test-phase2"
 }