On a `Term error from Cmdliner, print a Hint about how to pass a space

[hannesm]

See #1248 @samoht @patricoferris @pitag-ha

Of course, happy to improve the text. But since this is a pretty common issue, I'm happy to have such a hint.

[hannesm]

There's another way how to get it working: --hello='foo\ bar' (or the mentioned --hello='"foo bar"'). I'm undecided which one is easier to understand, and what the leading sentence should be. :)

[palainp]

I prefer 'foo\ bar' but it seems easier to use double quotes in shell scripts, so LGTM :)

[hannesm]

@palainp thanks. I updated the docstring. It now includes both options.

I put it in two lines to adhere to the 80 character limit.

[palainp]

Thank you !

[samoht]

I'm still not very satisfied we need to impose such a dance on the users. But this is a more general issue. This PR is a good change anyway :-) feel free to merge!

[hannesm]

And the issue is re-appearing every few years:

• Add single quote handling for unescaped strings parse-argv#6
• Quotes in hvt boot parameters get lost Solo5/solo5#281
• Escape boot parameters to quoted space-separated format of Parse_argv Solo5/solo5#283
• Command-line inputs: bad parsing? mirage-solo5#33
• Be sure to surround any arguments with double-quote mirage-bootvar-unix#2
• keys with spaces are not supported? #1248

And my opinion is: let's keep the amount of parsing at a minimum to the C land, and have all of it consistent across targets (otherwise it is really confusing).