fix(internal/godocfx): use exact list of top-level decls

[tbpg]

Here is a resulting diff from the firestore package:

-    content: var MergeAll <a href="#setoption">SetOption</a> = <a href="#merge">merge</a>{/*
-      contains filtered or unexported fields */}
+    content: var MergeAll <a href="#setoption">SetOption</a> = merge{/* contains filtered
+      or unexported fields */}

Note that merge should not be linked. This PR fixes that.

[codyoss]

LGTM