Add incantation to display feature in docs.rs

[CosmicHorrorDev]

This sets things up so that the frame module gets a marker in docs.rs that the "frame" feature has to be used. You can see this locally by running

$ RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features --open

[codecov[bot]]

Codecov Report

Merging #101 (1b88e69) into main (be2ea76) will decrease coverage by 0.99%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
- Coverage   89.39%   88.41%   -0.99%     
==========================================
  Files          11       11              
  Lines        2179     2192      +13     
==========================================
- Hits         1948     1938      -10     
- Misses        231      254      +23     

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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[PSeitz]

Nice, thanks!