Improve crypto/keys docs

[alessio]

In addition to #5822

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[golangcibot]

package comment should not have leading space (from golint)

[codecov]

Codecov Report

Merging #5830 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5830   +/-   ##
=======================================
  Coverage   32.30%   32.30%           
=======================================
  Files         353      353           
  Lines       39461    39461           
=======================================
  Hits        12749    12749           
  Misses      25439    25439           
  Partials     1273     1273           

[alessio]

superseded by #5833