Added crate feature requirements to docs

[AntonnMal]

Steam algorithm was hidden from docs and some functions were not marked as feature-locked.

Also, some notes that are outside of this PR:

• Shadowing feature-gated functions is probably a bad idea because they do not show up in docs and lead to inconsistent examples (e.g. secret.rs)
• Is digits being outside of the 6..=8 range unwanted behavior? If so, then why not use an enum?

Note: This change was not tested on docs.rs, only using local cargo:

cargo +nightly rustdoc --all-features -- --cfg docsrs

[constantoine]

Hi! Thank you for your PR. I just have a few question, but it looks good! I found some examples to update yesterday, after merging I'll probably fix those up too before publishing the next version

[constantoine]

I'll admit I've never went too far with rustdocs. This is nice to know, thank you :)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7873dc0) 94.78% compared to head (de61f0a) 94.87%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   94.78%   94.87%   +0.08%     
==========================================
  Files           6        6              
  Lines        1420     1443      +23     
==========================================
+ Hits         1346     1369      +23     
  Misses         74       74              

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