Clarify Gremlin-Groovy __. prefix guidance and datetime() return type

[spmallette]

This is a small, conservative documentation fix touching two reference sections.

Gremlin-Groovy differences (reference/gremlin-variants.asciidoc)

The section previously stated that as, in, and not are reserved words that "must always be prefixed with __.". The recommendation to use the __. prefix is kept, but the justification is reframed: rather than a hard reserved-word parser rule, the reason to be explicit is to get deterministic, portable resolution — use __. for an anonymous traversal step, and P. when you actually want a predicate.

This is clarified especially for not: a bare not(...) resolves to P.not(P) (predicate negation), whereas __.not(traversal) is the filter step. Being explicit (__.not(...) for the step, P.not(...) for the predicate) avoids the ambiguity.

A note is also added that datetime() returns a java.time.OffsetDateTime, which the section did not previously state.

Canonical Gremlin literal types (reference/the-traversal.asciidoc)

The DateTime row now notes that the all-lowercase datetime form is also accepted (the grammar accepts datetime, DateTime, and DATETIME).

Testing

Built the rendered documentation locally (bin/process-docs.sh) and reviewed the affected pages in the HTML output; all three changes render as intended.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.47%. Comparing base (a28cd1f) to head (a145110).
⚠️ Report is 438 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3615      +/-   ##
============================================
+ Coverage     76.35%   76.47%   +0.12%     
- Complexity    13424    14297     +873     
============================================
  Files          1012     1036      +24     
  Lines         60341    64620    +4279     
  Branches       7075     7664     +589     
============================================
+ Hits          46076    49421    +3345     
- Misses        11548    12108     +560     
- Partials       2717     3091     +374     

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