Clarify gremlin-javascript Limitations for Character/Duration and subgraph

[spmallette]

The gremlin-javascript Limitations section in docs/src/reference/gremlin-variants.asciidoc states two limitations correctly but leaves a reader unable to tell what actually happens when they are hit.

This change makes two of those bullets precise:

• Unsupported Character and Duration values. The bullet now names the failure mode: if the server returns one of these types the client throws a deserialization error for the unknown type code, rather than failing silently or coercing the value. It also notes that these types cannot currently be produced from the client, since GremlinLang provides no literal syntax for them, so the failure is only observable against a server that emits them directly.
• The subgraph() detached graph. The previous wording said the result "cannot be passed to traversal().with(...)", which is imprecise: passing it does not raise an error immediately. The bullet now explains that it yields a non-functional traversal source that fails only when a traversal is executed against it (surfacing as an opaque this.connection.submit is not a function).

No behavior changes. Docs-only precision fix, scoped to this section; other variant sections are untouched. The docs render cleanly.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.43%. Comparing base (a28cd1f) to head (e8c7efa).
⚠️ Report is 348 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3557      +/-   ##
============================================
+ Coverage     76.35%   76.43%   +0.07%     
- Complexity    13424    14284     +860     
============================================
  Files          1012     1036      +24     
  Lines         60341    64561    +4220     
  Branches       7075     7656     +581     
============================================
+ Hits          46076    49349    +3273     
- Misses        11548    12122     +574     
- Partials       2717     3090     +373     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.