refactor: add comments about `WeakSession` #8205

Job	Run time
Unit tests on ubuntu-latest	18m 44s
Lints and doc tests on ubuntu-latest	1m 13s
Typos Check	6s
Unit tests on windows-latest	28m 0s
Unit tests on macos-13	14m 15s
Lints and doc tests on windows-latest	3m 47s
Unit tests on macos-14	15m 7s
Lints and doc tests on macos-13	2m 8s
Lints and doc tests on macos-14	1m 11s
Memory leak checks	0s
CI status checks	1s
	1h 24m 32s

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