docs: explain why | is an unconditional tag-variable terminator

[DylanPiercey]

Documents why | must stay an unconditional terminator in the tag-variable scanners, so the "parenthesize your union" requirement is not re-investigated as a parser bug.

| is overloaded — it is both the TS union separator and the tag-body-parameters delimiter. Unlike < (which returns !expression.inType because generic brackets are balanced), | cannot be made type-aware: a typed tag variable can be immediately followed by body params, e.g.

<my-list/ref: Api|item, index|>...</my-list>

which must parse as tag variable ref: Api + params item, index. Making | non-terminating inside a type annotation swallows the |params| into the type as a union (this was tried in #230 and reverted). A union type annotation must therefore be parenthesized: (string | null).

Comment-only; no behavior change, no snapshot changes.

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Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.92%. Comparing base (93850d2) to head (a58a797).

Files with missing lines	Patch %	Lines
src/states/OPEN_TAG.ts	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
- Coverage   99.95%   99.92%   -0.03%     
==========================================
  Files          34       34              
  Lines        4270     4274       +4     
  Branches      793      793              
==========================================
+ Hits         4268     4271       +3     
- Misses          2        3       +1     

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[DylanPiercey]

Closing — the equivalent decision comment now lives at the marko compiler consumer (parser.js onTagVar, merged in marko-js/marko#3506), which is where an agent investigating the EOF reached while parsing expression failure lands. That breadcrumb is sufficient; no need for a duplicate here.