Add toolchain type or Java bootstrap runtime

The `bootclasspath` rule in `rules_java` will soon use this toolchain
type instead of the regular Java runtime toolchain type, which naturally
carries a constraint on the target platform.

Also adds more detailed explanations of the now three Java toolchain
types.

tools/jdk/BUILD.tools

@@ -7,12 +7,42 @@ exports_files([
     "java_stub_template.txt",
 ])
 
-# Used to distinguish toolchains used for Java development, ie the JavaToolchainProvider.
+# A single binary distribution of a JDK (e.g., OpenJDK 17 for Windows arm64) provides three
+# different types of toolchains from the perspective of Bazel:
+
+# The compilation toolchain, which provides the Java runtime used to execute the Java compiler, as
+# well as various helper tools and settings.
+#
+# Toolchains of this type typically have constraints on the execution platform so that their Java
+# runtime can run the compiler, but not on the target platform as Java compilation outputs are
+# platform independent.
+#
+# Obtain the associated JavaToolchainInfo via:
+#   ctx.toolchains["@bazel_tools//tools/jdk:toolchain_type"].java
 toolchain_type(name = "toolchain_type")
 
-# Used to distinguish toolchains used for Java execution, ie the JavaRuntimeInfo.
+# The Java runtime that executable Java compilation outputs (e.g., java_binary with
+# create_executable = True) will run on.
+#
+# Toolchains of this type typically have constraints on the target platform so that the runtime's
+# native 'java' binary can be run there, but not on the execution platform as building an executable
+# Java target only requires copying or symlinking the runtime, which can be done on any platform.
+#
+# Obtain the associated JavaRuntimeInfo via:
+#   ctx.toolchains["@bazel_tools//tools/jdk:runtime_toolchain_type"].java_runtime
 toolchain_type(name = "runtime_toolchain_type")
 
+# The Java runtime to extract the bootclasspath from that is then used to compile Java sources.
+#
+# As the bootclasspath is platform independent, toolchains of this type may have no constraints.
+# Purely as an optimization to prevent unnecessary fetches of remote runtimes for other
+# architectures, toolchains of this type may have constraints on the execution platform that match
+# those on the corresponding compilation toolchain.
+#
+# Toolchains of this type are only consumed internally by the bootclasspath rule and should not be
+# accessed from Starlark.
+toolchain_type(name = "bootstrap_runtime_toolchain_type")
+
 # Aliases value of --plugins flag as a JavaPluginInfo
 java_plugins_flag_alias(
     name = "java_plugins_flag_alias",