docs: stop pointing command help at deprecated flags

What does this MR do and why?

Two commands advertise flags that the very same file deprecates, so anyone following the built-in help is steered onto a deprecated path.

glab issue list / glab incident list

internal/commands/issuable/list/issuable_list.go ends its examples with:

glab issue list --milestone release-2.0.0 --opened

but a few lines further down the same file does:

_ = issueListCmd.Flags().MarkHidden("opened")
_ = issueListCmd.Flags().MarkDeprecated("opened", "default if --closed is not used.")

So --opened does not appear in --help at all, and copying the example prints a deprecation warning while changing nothing, because open is already the default.

glab mr note

Its description reads "Use --resolve or --unresolve to manage existing discussion threads". Both flags are deprecated in favour of the subcommands glab mr note resolve and glab mr note reopen, and neither subcommand was mentioned in the description.

This MR drops --opened from the examples and points the mr note description at the subcommands that replaced the flags. No flag is added or removed, so nothing changes for anyone still passing them.

docs/source is regenerated with make gen-docs in the same commit.

How to reproduce

glab issue list --milestone release-2.0.0 --opened
# Flag --opened has been deprecated, default if --closed is not used.

glab mr note --help
# suggests --resolve / --unresolve, neither of which is listed under FLAGS

Related issues

Closes #8408