clarify user ns mappings and time ns offset configurations

[lifubang]

Nowadays, when we let the container to join an existing user or time namespace, we should not provide uidMappings, gidMappings, and timeOffsets configurations.
Let's add some description to clarify user ns mappings and time ns offset configurations.

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background:

1. (u|g)idMappings should not exist when joining an existing user ns runc#4122
2. libcontainer: remove all mount logic from nsexec runc#3985 (comment)