Add extra comments about extension fields

Author: Juan Tejada

packages/react-devtools-extensions/src/__tests__/updateMockSourceMaps.js

@@ -133,10 +133,15 @@ function compile(fileName) {
   const encodedHookMap = generateEncodedHookMap(parsed);
   const fbSourcesExtendedSourceMap = {
     ...sourceMap,
+    // When using the x_fb_sources extension field, the first item
+    // for a given source is reserved for the Function Map, and the
+    // Hook Map is added as the second item.
     x_fb_sources: [[null, encodedHookMap]],
   };
   const reactSourcesExtendedSourceMap = {
     ...sourceMap,
+    // When using the x_react_sources extension field, the first item
+    // for a given source is reserved for the Hook Map.
     x_react_sources: [[encodedHookMap]],
   };
 

packages/react-devtools-extensions/src/parseHookNames/parseHookNames.js

@@ -667,8 +667,15 @@ function extractHookMapFromSourceMap(
 ): HookMap | null {
   let hookMap;
   if (sourcemap.hasOwnProperty(REACT_SOURCES_EXTENSION_KEY)) {
+    // When using the x_react_sources extension field, the first item
+    // for a given source is reserved for the Hook Map, which is why
+    // we look up the index at position 0.
     hookMap = sourcemap[REACT_SOURCES_EXTENSION_KEY][sourceIndex][0];
   } else if (sourcemap.hasOwnProperty(FB_SOURCES_EXTENSION_KEY)) {
+    // When using the x_fb_sources extension field, the first item
+    // for a given source is reserved for the Function Map, and the
+    // Hook Map is added as the second item, which is why we look up
+    // the index at position 1.
     hookMap = sourcemap[FB_SOURCES_EXTENSION_KEY][sourceIndex][1];
   }
   if (hookMap != null) {