Language extension API docs

Add docs for the language extension API.

Drive-by: Fix a comment on the API typings.

Bug: none
Change-Id: I411845a4bff8d1ecbfec41875e998234b8981b80
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/4110802
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Auto-Submit: Philip Pfaffe <pfaffe@chromium.org>
Commit-Queue: Philip Pfaffe <pfaffe@chromium.org>
diff --git a/extension-api/ExtensionAPI.d.ts b/extension-api/ExtensionAPI.d.ts
index 31e16b0..2daaa26 100644
--- a/extension-api/ExtensionAPI.d.ts
+++ b/extension-api/ExtensionAPI.d.ts
@@ -273,7 +273,8 @@
       }>;
 
       /**
-       * Find locations in source files from a location in a raw module
+       * Retrieve function name(s) for the function(s) containing the rawLocation. This returns more than one entry if
+       * the location is inside of an inlined function with the innermost function at index 0.
        */
       getFunctionInfo(rawLocation: RawLocation):
           Promise<{frames: Array<FunctionInfo>}|{missingSymbolFiles: Array<string>}>;