improve module docstrings #3172

Job	Run time
pjrt-linux-x86_64	10s
build-tests-xla-cpu-darwin-aarch64	15m 36s
build-tests-xla-cpu-linux-x86_64	2m 28s
build-tests-xla-cuda-linux-x86_64	2m 30s
pjrt-plugin-xla-cpu-darwin-aarch64	12s
pjrt-plugin-xla-cpu-linux-x86_64	8s
pjrt-plugin-xla-cuda-linux-x86_64	10s
shellcheck	3s
pjrt-darwin-aarch64	8s
readme	1m 28s
tutorials	1m 13s
test-xla-cpu-linux-x86_64	1m 36s
test-xla-cuda-linux-x86_64	1m 15s
test-xla-cpu-darwin-aarch64	2m 46s
	29m 43s

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