Fix/doc releases

[DavisVaughan]

• Adds a number of updates / fixes to the Release endpoint docs

• fredr_release_tables() now uses tibble::enframe() to clean up the top level output.

[codecov-io]

Codecov Report

Merging #43 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files          35       35           
  Lines         665      665           
=======================================
  Hits          663      663           
  Misses          2        2

Impacted Files	Coverage Δ
R/fredr_release_sources.R	100% <ø> (ø)	⬆️
R/fredr_release_tables.R	100% <ø> (ø)	⬆️
R/fredr_release_dates.R	100% <ø> (ø)	⬆️
R/fredr_releases_dates.R	100% <ø> (ø)	⬆️
R/fredr_release_series.R	100% <ø> (ø)	⬆️
R/fredr_release_related_tags.R	100% <ø> (ø)	⬆️
R/fredr_release_tags.R	100% <ø> (ø)	⬆️
R/fredr_release.R	100% <ø> (ø)	⬆️
R/fredr.R	100% <100%> (ø)	⬆️

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