Clarify group()-step callouts for non-productive key vs value projection

[spmallette]

The two group()-step examples in reference/the-traversal.asciidoc that project age first as a key and then as a value were annotated with symmetric callouts, both saying the non-productive entries are "filtered". Their runtime behavior is actually asymmetric:

• A non-productive key-projection (by('age').by('name')) drops the vertex from the result map entirely — no key is produced for vertices lacking an age value.
• A non-productive value-projection (by('name').by('age')) keeps the key (since name is productive for every vertex) but leaves its value list empty, e.g. ripple=[], lop=[].

The old symmetric wording made the {ripple:[], lop:[]} output surprising. This change rewrites callout <2> to explain that keys are retained with empty value lists, and updates callout <1> to state the entry is dropped entirely, drawing an explicit contrast between the two cases. The productive cross-reference is preserved.

Only the two callouts changed; the example code lines are untouched and still render their live output at build time, which confirms the reworded prose matches what the reader sees.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.50%. Comparing base (a28cd1f) to head (89a62a9).
⚠️ Report is 373 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3572      +/-   ##
============================================
+ Coverage     76.35%   76.50%   +0.14%     
- Complexity    13424    14300     +876     
============================================
  Files          1012     1036      +24     
  Lines         60341    64608    +4267     
  Branches       7075     7661     +586     
============================================
+ Hits          46076    49431    +3355     
- Misses        11548    12088     +540     
- Partials       2717     3089     +372     

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