Add a few missing docstrings for API docs

[c42f]

Fixes #300

[codecov[bot]]

Codecov Report

Merging #301 (ff0b896) into main (f6431b8) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #301   +/-   ##
=======================================
  Coverage   96.80%   96.80%           
=======================================
  Files          14       14           
  Lines        4094     4094           
=======================================
  Hits         3963     3963           
  Misses        131      131           

Impacted Files	Coverage Δ
src/parse_stream.jl	96.42% <ø> (ø)
src/source_files.jl	97.36% <ø> (ø)
src/syntax_tree.jl	95.13% <ø> (ø)