Document the language selection process of gdef

[ian-flores]

This PR addresses issue #14 brought up by @beatrizmilz to the effects of adding documentation for the language selection process in the gdef function.

[codecov]

Codecov Report

Merging #24 (7f55ea2) into main (14d4e15) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage   45.67%   45.67%           
=======================================
  Files          11       11           
  Lines         370      370           
=======================================
  Hits          169      169           
  Misses        201      201           

Impacted Files	Coverage Δ
R/entry.R	41.90% <0.00%> (ø)
R/gdef.R	81.81% <ø> (ø)
R/parse.R	42.85% <ø> (ø)

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