Clarify subgraph-step TinkerVertex caveat as implementation-dependent

[spmallette]

The subgraph()-step section cautions that a TinkerGraph subgraph's elements (TinkerVertex/TinkerEdge) can not be used directly in Gremlin running against the original graph, and presents this example as one that "would likely return an error":

Vertex v = sg.V().has('name','marko').next();
List<Vertex> vertices = g.V(v).out().toList();

For the example as shown, though, the original graph is also a TinkerGraph. In that case TinkerGraph looks vertices up by their identifier, so the reference resolves and g.V(v).out() returns the expected result rather than erroring. The warning is accurate only when the original graph is a different implementation that may not recognize a foreign TinkerVertex.

This reduces the caveat paragraph and its callout to implementation-dependent framing:

• When the original graph is also a TinkerGraph (as in the modern graph example shown), the reference resolves by identifier and the traversal succeeds.
• When the original graph is a different implementation, it may reject a foreign TinkerVertex and could return an error.

Scope is limited to the caveat paragraph and its second callout. The edge-induced subgraph examples above and the ReferenceVertex/id() portable-recommendation block below are unchanged, and the illustrative snippet remains non-executed text. The reference book was rendered locally to confirm it builds and reads coherently.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.52%. Comparing base (a28cd1f) to head (46f0a0e).
⚠️ Report is 373 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3571      +/-   ##
============================================
+ Coverage     76.35%   76.52%   +0.16%     
- Complexity    13424    14309     +885     
============================================
  Files          1012     1036      +24     
  Lines         60341    64608    +4267     
  Branches       7075     7661     +586     
============================================
+ Hits          46076    49442    +3366     
- Misses        11548    12078     +530     
- Partials       2717     3088     +371     

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