API: Add missing @return Javadoc to fluent/namespace method

[uros-b]

Several public API methods are missing the @return Javadoc tag that their sibling methods document. This adds it for consistency and completeness. Javadoc-only; no signature or behavior change.

[uros-b]

@nastra please review

[nastra]

I did a quick analysis and we have the same pattern in, so we might want to update those places as well to be consistent

• util/BinaryUtil#truncateBinary(ByteBuffer, int)
• util/BinaryUtil#truncateBinaryUnsafe(ByteBuffer, int)
• io/FileIO#properties()
• formats/ReadBuilder#split(long, long), #caseSensitive(boolean), #filter(Expression) — siblings set/setAll already say "@return this for method chaining"
• TrackingBuilder#added, #from, #deleted, #replaced
• TrackedFile#copyWithStats(Set)
• MetricsConfig#forTable(Table)
• ManifestFilterManager#buildSummary(Iterable)
• MergingSnapshotProducer#addedDeleteFiles(...)
• data/avro/RawDecoder#create(...)
• encryption/NativeFileCryptoParameters#create(ByteBuffer)
• io/PartitionedWriter#partition(T) and io/PartitionedFanoutWriter#partition(T)