Add documentation for clangmetatool::IncludeGraphDependencies

Signed-off-by: Vaibhav Yenamandra <vyenamandra@bloomberg.net>

Author: envp

include/clangmetatool/include_graph_dependencies.h

@@ -4,6 +4,11 @@
 #include <clangmetatool/collectors/include_graph_data.h>
 
 namespace clangmetatool {
+
+/**
+ * Collect stateless functions to query and and modify the state of
+ * dependencies of a given `clangmetatool::IncludeGraphData` structure.
+ */
 struct IncludeGraphDependencies {
   /**
    * Decrement the reference count tracked for this edge, notifying the caller