docs: Auto-document features on docs.rs + improved wording for the top-level documentation

[frol]

Currently, the global-contract feature it hard to discover as while the methods are visible in the docs:

There is no indication that global-contracts feature is required. See tokio as example:

To enable this documentation, we just need to enable the nightly feature and hide it behind the conditional compilation flag for docsrs: #![cfg_attr(docsrs, feature(doc_auto_cfg))]

https://docs.rs/about/builds#detecting-docsrs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.72%. Comparing base (04515b4) to head (736fdd2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1386   +/-   ##
=======================================
  Coverage   80.72%   80.72%           
=======================================
  Files         104      104           
  Lines       15462    15462           
=======================================
  Hits        12482    12482           
  Misses       2980     2980           

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