Fix copy-paste leftovers in public API javadoc

[thswlsqls]

No issue: docs-only javadoc fix, submitting directly.

Description

Six javadoc lines in stable public API describe something the method does not do. Each fix reuses a sibling member's wording.

• DoubleHistogramBuilder#ofLongs: "Sets the Counter" → Histogram (cf. DoubleGaugeBuilder#ofLongs).
• LongUpDownCounterBuilder#ofDoubles: "Sets the Counter" → UpDownCounter (copied from LongCounterBuilder#ofDoubles, where it is correct).
• TraceId#fromBytes: "first 26 bytes" → 16, matching BYTES_LENGTH and the @param right below (cf. SpanId#fromBytes, "first 8 bytes").
• TraceFlags#isSampled: a * trapped inside {@code} rendered as otherwise * false (cf. isTraceIdRandom, same file).
• TraceFlags#asHex: @return said "the byte representation", copied from asByte; returns String.
• TraceId#fromLongs: summary said "the bytes (16-byte array) representation"; returns String, and its own @return already says hex (cf. SpanId#fromLong).

Testing done

• Docs-only, test-exempt: no test added.
• ./gradlew :api:all:check passed.
• ./gradlew :api:all:javadoc --rerun-tasks: the stray * is gone from TraceFlags.html, and TraceId.html now reads "lowercase hex (base16) representation" for fromLongs.
• No apidiff change: javadoc does not reach bytecode.
• Not run: *IT and GraalVM native-image tests (outside check).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.62%. Comparing base (dd811c4) to head (baf2d6b).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8693   +/-   ##
=========================================
  Coverage     91.62%   91.62%           
  Complexity    10327    10327           
=========================================
  Files          1003     1003           
  Lines         27138    27138           
  Branches       3187     3187           
=========================================
  Hits          24866    24866           
  Misses         1566     1566           
  Partials        706      706           

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