docs: [chat] Update doc for `SetUpSpace` in service `ChatService` to support group members

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docs: Update doc for CreateMembership in service ChatService to support group members docs: Update doc for field group_member in message google.chat.v1.Membership

PiperOrigin-RevId: 646951938

Source-Link: https://github.com/googleapis/googleapis/commit/ac0973e85be447cb4eadd58e14d1e83c1699b4be

Source-Link: https://github.com/googleapis/googleapis-gen/commit/2b187e29a3d1904d3e5981bb3c8b7ab70a002393 Copy-Tag: eyJwIjoicGFja2FnZXMvZ29vZ2xlLWNoYXQvLk93bEJvdC55YW1sIiwiaCI6IjJiMTg3ZTI5YTNkMTkwNGQzZTU5ODFiYjNjOGI3YWI3MGEwMDIzOTMifQ==

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