Clarify the comments on invalid/table/injection-* tests

Not super-clear *why* these are invalid at a glance, and that issue is
long. Summarize the issue in a comment.

tests/invalid/table/injection-1.toml

@@ -1,6 +1,17 @@
+# First a.b.c defines a table: a.b.c = {z=9}
+#
+# Then we define a.b.c.t = "str" to add a str to the above table, making it:
+#
+#   a.b.c = {z=9, t="..."}
+#
+# While this makes sense, logically, it was decided this is not valid TOML as
+# it's too confusing/convoluted.
+# 
+# See: https://github.com/toml-lang/toml/issues/846
+#      https://github.com/toml-lang/toml/pull/859
+
 [a.b.c]
   z = 9
+
 [a]
   b.c.t = "Using dotted keys to add to [a.b.c] after explicitly defining it above is not allowed"
-
-# see https://github.com/toml-lang/toml/issues/846

tests/invalid/table/injection-2.toml

@@ -1,6 +1,8 @@
+# This is the same issue as in injection-1.toml, except that nests one level
+# deeper. See that file for a more complete description.
+
 [a.b.c.d]
   z = 9
+
 [a]
   b.c.d.k.t = "Using dotted keys to add to [a.b.c.d] after explicitly defining it above is not allowed"
-
-# see https://github.com/toml-lang/toml/issues/846