Enable doc_cfg on docsrs feature

[PhilippGackstatter]

Description of change

Fixes the failing build of identity_iota on docs.rs. This command succeeds now:

RUSTDOCFLAGS='--cfg docsrs' cargo +nightly doc --all-features --no-deps --open

Note that #![cfg_attr(docsrs, feature(doc_cfg))] will need to be added to all crates where we start to use the #[cfg_attr(docsrs, doc(cfg(feature = "xyz")))] feature.

Links to any relevant issues

n/a

Type of change

Add an x to the boxes that are relevant to your changes.

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Built the docs successfully.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[eike-hass]

We also need to merge this into the support/0.6 branch.