docs: clarify max_num_elements behavior

[sapunyangkut]

Summary

• clarify that max_num_elements limits byte ranges in one XRootD vector read
• point readers to entry_stop when they want to limit the number of TTree entries read

Closes #1429.

Validation

• python -m compileall -q src/uproot/reading.py
• imported uproot.open and asserted that the rendered docstring contains the new clarification and cross-reference
• built a focused Sphinx autodoc page for uproot.open; the patched docstring produced the same 14 pre-existing warnings as the unmodified baseline and no new warning

The full repository Sphinx build was also attempted. On Windows it stops in the existing docs-sphinx/prepare_docstrings.py path handling because it searches for uproot/ in a backslash-based __file__ path; this is unrelated to the documentation change.

AI assistance

OpenAI Codex was used to investigate the issue, draft the wording, and run local validation. The final diff and validation evidence were reviewed by the contributor before submission.

[ariostas]

Thank you @sapunyangkut! I suggested a small tweak. Also, please make the similar modifications to uproot.interate and uproot.concatenate.