Use doc_auto_cfg and remove manual doc(cfg()) gates

[ctz]

Also remove now-duplicated comments about what features an item requires.

[codecov]

Codecov Report

Merging #150 (7d9ddec) into main (7bdf289) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #150   +/-   ##
=======================================
  Coverage   96.21%   96.21%           
=======================================
  Files          15       15           
  Lines        4304     4304           
=======================================
  Hits         4141     4141           
  Misses        163      163           

Files Changed	Coverage Δ
src/crl.rs	99.53% <ø> (ø)
src/end_entity.rs	100.00% <ø> (ø)
src/error.rs	50.00% <ø> (ø)
src/ring_algs.rs	100.00% <ø> (ø)
src/signed_data.rs	99.18% <ø> (ø)
src/subject_name/dns_name.rs	86.87% <ø> (ø)
src/subject_name/ip_address.rs	98.37% <ø> (ø)
src/time.rs	96.15% <ø> (ø)

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[cpu]

Thx!