chore: document the reason for symbol.for

[dyladan]

Documenting the reason for Symbol.for() in the code so that the reason we break from the spec is clear to new users and so we don't accidentally change to use Symbol() in the future.

[codecov]

Codecov Report

Merging #64 (acdb9b4) into main (6dda822) will not change coverage.
The diff coverage is n/a.

❗ Current head acdb9b4 differs from pull request most recent head 5de6aa8. Consider uploading reports for the commit 5de6aa8 to get more accurate results

@@           Coverage Diff           @@
##             main      #64   +/-   ##
=======================================
  Coverage   94.69%   94.69%           
=======================================
  Files          40       40           
  Lines         509      509           
  Branches       80       80           
=======================================
  Hits          482      482           
  Misses         27       27           

Impacted Files	Coverage Δ
src/context/context.ts	82.35% <ø> (ø)

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