Ping rustdoc team members on changes to rustdoc-json-types

CraftSpider graciously volunteered :) but the main point of this change
is to have extra attention called to the fact that it's changing, since
almost any change is a breaking change.

Author: jyn514

highfive/configs/rust-lang/rust.json

@@ -67,6 +67,10 @@
         "src/tools/tidy":           ["@Mark-Simulacrum"]
     },
     "mentions": {
+        "src/rustdoc-json-types": {
+          "message": "rustdoc-json-types is a **public** (although nightly-only) API. Consider changing `src/librustdoc/json/conversions.rs` instead; otherwise, make sure you update `format_version`.",
+          "reviewers": ["@CraftSpider", "@aDotInTheVoid"]
+        },
         "src/librustdoc/html/static": {
             "message": "Some changes occurred in HTML/CSS/JS.",
             "reviewers": ["@GuillaumeGomez"]