Document glibc NSS plugin limitations, especially for DNS resolution

[twz123]

Spinned off from #3351 (comment):

The docs do not cover the fact that the official k0s binaries cannot rely on the glibc NSS plugins for name resolution (among other things). We should add some notes about this in the external runtime dependencies section of the docs, and maybe also include a link to those docs somewhere in the k0s sysinfo command.

[github-actions]

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