Update rust-feature-support.md

[celinval]

Description of changes:

Remove the part about not being able to link std library functions.

Resolved issues:

N/A

Related RFC:

N/A

Call-outs:

Testing:

• How is this change tested?

• Is this a refactor change?

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.