Document crosstool_top a little better.

--
MOS_MIGRATED_REVID=90612869

tools/cpp/BUILD

diff --git a/tools/cpp/BUILD b/tools/cpp/BUILD
index 2721e32..f35d2bb 100644
--- a/tools/cpp/BUILD
+++ b/tools/cpp/BUILD
@@ -18,6 +18,16 @@
     srcs = [],
 )
 
+# This is the entry point for --crosstool_top.  Toolchains are found
+# by lopping off the name of --crosstool_top and searching for
+# "cc-compiler-${CPU}" in this BUILD file, where CPU is the target CPU
+# specified in --cpu.
+#
+# This file group should include
+#   * all cc_toolchain targets supported
+#   * all file groups that said cc_toolchain might refer to,
+# including the default_grte_top setting in the CROSSTOOL
+# protobuf.
 filegroup(
     name = "toolchain",
     srcs = [