fix(completions): don't wrap completion item help in parenthesis

[jakobhellermann]

What does this PR try to resolve?

Fish shell automatically wraps the help text in parenthesis, so this wil result in them being applied twice.

On zsh it previously looked like this

--target-dir                          -- Directory for all generated artifacts
--timings                             -- Timing output formats (unstable) (comma separated): html, json
--unit-graph                          -- Output build graph in JSON (unstable)
--verbose                             -- Use verbose output (-vv very verbose/build.rs output)
-Z                                    -- Unstable (nightly-only) flags to Cargo, see 'cargo -Z help' for details
fix-json                              -- (from rustfix)
matches                               -- (from cargo-platform)
stdout-redirected      file-provider  -- (from cargo-credential)

and will now be

fix-json                              -- from rustfix
matches                               -- from cargo-platform
stdout-redirected      file-provider  -- from cargo-credential

Bash doesn't show the help text at all so it doesn't matter.

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[jakobhellermann]

(the fact that all the other flags are being completed at all in the middle of a flag accepting a value is due to the fact that we do optional arguments as .num_args(0..=1) where the 0 case is handled manually. I'll open another issue for that)

[epage]

(the fact that all the other flags are being completed at all in the middle of a flag accepting a value is due to the fact that we do optional arguments as .num_args(0..=1) where the 0 case is handled manually. I'll open another issue for that)

See #14606