test,doc: cover and document multi-byte offset/size in randomFill

[kyungrae2002]

test-crypto-random.js verified offset/size placement only for
single-byte buffers (Buffer / Uint8Array), so the element-to-byte
conversion for multi-byte typed arrays was never exercised. The docs for
randomFill() / randomFillSync() documented the size default only as
buffer.length - offset, which is meaningless for ArrayBuffer / DataView
(no .length), and never stated the offset/size unit.

• Document that offset/size are in elements for a TypedArray and in
  bytes for an ArrayBuffer / DataView, with the default given per type
• Add regression tests filling one element of a Uint16Array / Uint32Array
  at a non-zero offset, asserting neighboring elements are untouched

No runtime behavior changes.

[nodejs-github-bot]

Review requested:

• @nodejs/crypto

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.16%. Comparing base (ed63b19) to head (8fc10c8).
⚠️ Report is 165 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64834      +/-   ##
==========================================
- Coverage   92.01%   90.16%   -1.86%     
==========================================
  Files         381      746     +365     
  Lines      170167   242764   +72597     
  Branches    26075    45739   +19664     
==========================================
+ Hits       156585   218877   +62292     
- Misses      13291    15387    +2096     
- Partials      291     8500    +8209     

see 532 files with indirect coverage changes

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