Add comments & remove unneeded accessor on static flatbuffer API

* Add comments indicating that FromFlatbuffer is a "deep" copy, to make
  clear that users do not need to manually copy sub-objects themselves.
* Remove the AsMutableFlatbufferVector method. Providing non-const
  access to the raw flatbuffer types does not seem to have any value,
  and it was resulting in people attempting to use the wrong API to
  mutate the flatbuffer state.

Change-Id: Ic3160293a0e4d4d0c439b387cc5dcbfe2e02cd11
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/flatbuffers/static_flatbuffers.cc b/aos/flatbuffers/static_flatbuffers.cc
index 019afda..8d8942c 100644
--- a/aos/flatbuffers/static_flatbuffers.cc
+++ b/aos/flatbuffers/static_flatbuffers.cc
@@ -471,6 +471,8 @@
       R"code(
   // Copies the contents of the provided flatbuffer into this flatbuffer,
   // returning true on success.
+  // This is a deep copy, and will call FromFlatbuffer on any constituent
+  // objects.
   [[nodiscard]] bool FromFlatbuffer(const Flatbuffer *other) {
     Clear();
     %s