docs: pass resource generation date arguments by keyword

[qorud02]

The Resource Generation examples show the three date options as a single positional argument. In download_generate_resource, that position is actually columns, so adapting the example with a date column silently forwards it as the column selection while leaving datecol unset. The positional form in generate_resource also puts a year column or date callback in the datecol slot.

Use explicit datecol, yearcol, or date_function keywords in the examples, and explain that only one should be supplied. The two snippets now follow the current method signatures.

Validation:

• Ran the current Dataset.download_generate_resource method with a local downloader and a capture dataset. The original positional "date" was forwarded as columns="date", datecol=None; the corrected keyword was forwarded as columns=None, datecol="date".
• All 10 checks passed: four argument-forwarding checks and six signature-binding checks covering the three date keywords in both methods.
• git diff --check passes. This is a documentation-only change; resource generation and live HDX integration were not exercised.