Qualify rust trait template comments

[dani-garcia]

The doc links for parameter and error structs in the reqwest-trait template were copied over from the reqwest template, but they are not valid as-is, as these functions are now part of a trait and not standalone functions. This means for them to work correctly in IDEs and docs you need to qualify them with the trait name.

For example, VSCode, shows the link with a different color and displays a preview of the function, plus clicking on the name would navigate to the function.
With this change:

Without this change:

I've also removed the trailing whitespace from the template file, let me know if you prefer that to be reverted and split into a separate PR.

Tagging openapi-generator Rust committee, like the checklist mentions: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro

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[wing328]

lgtm. thanks for the contribution