clarification on -C option

[gjduine]

Closes #1129

For novice users, the addition of an e-mail address is unclear, so some additional information would be helpful

Full issue here:
#1121

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[martinosorb]

Thank you for this. It looks good in principle, but I find it a little dry: for example it's not clear to me what "hostnames, or no comment at all" means. Maybe we can make it into a friendlier sentence? Something like
"In the command above, the `-C- flag is used to label the SSH key with the owner's email address. It's good practice to use the same email associated to your GitHub account, but it does not have to be, and will not be used to validate the access. You can also decide not to add any label."

How does that sound?