Warn about dragons in the Bazel include path (#927)

https://github.com/google/glog/issues/837 has caused a couple of
disappointments for PR authors, so I'm hoping this comment can save them
some time, or even help them towards finding a complete solution for the
problem.

bazel/glog.bzl

@@ -201,6 +201,14 @@ def glog_library(namespace = "google", with_gflags = 1, **kwargs):
             ":stl_logging_h",
             ":vlog_is_on_h",
         ],
+        # https://github.com/google/glog/issues/837: Replacing
+        # `strip_include_prefix` with `includes` would avoid spamming
+        # downstream projects with compiler warnings, but would also leak
+        # private headers like stacktrace.h, because strip_include_prefix's
+        # implementation only creates symlinks for the public hdrs. I suspect
+        # the only way to avoid this is to refactor the project including the
+        # CMake build, so that the private headers are in a glog_internal
+        # subdirectory.
         strip_include_prefix = "src",
         defines = final_lib_defines,
         copts = final_lib_copts,