Explain --insecure flag better to avoid confusions

Description of problem:

Documentation Link: https://docs.openshift.com/container-platform/4.10/installing/installing_bare_metal/installing-restricted-networks-bare-metal.html#installation-user-infra-machines-coreos-installer-options_installing-restricted-networks-bare-metal

Section:
Table 12. coreos-installer subcommands, command-line options, and arguments
```
--insecure : Skip signature verification.
```

This '--insecure' flag needs to be explained better.

This BZ is a request to improve a bit the documentation of the --insecure flag, as nowadays it only says "Skip signature verification" which is a bit misleading. Probably "Skip RHCOS image signature verification" would be more clear.

Support has seen an area of improvement here because we faced a ticket where the use of this flag was not clear.

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A few other platform docs also have this table that explains - 'coreos-installer subcommands, command-line options, and arguments' that would require improvement. (not just the aforementioned doc)