Hparams: fix documentation of session and session group #10903

Job	Run time
lint-frontend	1m 41s
lint-misc	14s
lint-python-flake8 (3.9)	26s
build-data-server-pip (native, ubuntu-22.04, 1.65.0)	2m 19s
lint-python-flake8 (3.10)	23s
build-data-server-pip (native, macos-12, 1.65.0)	3m 12s
lint-python-flake8 (3.11)	17s
lint-python-yaml-docs	36s
build-data-server-pip (universal, ubuntu-22.04, 1.65.0)	15s
lint-rust (1.65.0, 0.16.1)	48s
build (tf, 3.9)	14m 30s
build (notf, 3.9)	10m 24s
	35m 5s

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