Add docs on guards

lib/rdf/guards.ex

@@ -1,7 +1,17 @@
 defmodule RDF.Guards do
+  @moduledoc """
+  A collection of guards.
+  """
+
   import RDF.Utils.Guards
 
+  @doc """
+  Returns if the given value is an atom which could potentially be an `RDF.Vocabulary.Namespace` term.
+  """
   defguard maybe_ns_term(term) when maybe_module(term)
 
+  @doc """
+  Returns if the given value is a triple, i.e. a tuple with three elements.
+  """
   defguard is_triple(t) when is_tuple(t) and tuple_size(t) == 3
 end

lib/rdf/utils/guards.ex

@@ -1,4 +1,8 @@
 defmodule RDF.Utils.Guards do
+  @moduledoc !"""
+             A collection of guards intended for internal use.
+             """
+
   defguard is_ordinary_atom(term)
            when is_atom(term) and term not in [nil, true, false]