comment about toolchain registration with :all

FIX @shs96c

Author: alexeagle

examples/MODULE.bazel

@@ -25,6 +25,13 @@ use_repo(protoc, "com_google_protobuf", "toolchains_protoc_hub")
 
 register_toolchains("@toolchains_protoc_hub//:all")
 
+# NB: the `:all` here is critical, because `proto_lang_toolchain` expands into two targets:
+# - proto_lang_toolchain rule [name]
+# - toolchain rule [name]_toolchain
+# and the second one is valid for registration.
+# Attempting to register a proto_lang_toolchain rule as a toolchain gives a baffling error like 
+# Misconfigured toolchains: //tools:protoc_java_toolchain is declared as a toolchain but has inappropriate dependencies.
+# Declared toolchains should be created with the 'toolchain' rule and should not have dependencies that themselves require toolchains.
 register_toolchains("//tools/toolchains:all")
 
 ####### PYTHON ##########