Reconcile the contradicting ICS field docs

[edalzell]

DOCUMENTATION.md documents the ICS download fields twice and the two lists disagree. Line 45 lists address, coordinates, description. Line 266, under events:download_link, lists location, description, link. Neither matches what the code actually does.

Actual 6.x behaviour, from Event::eventUrl() and Event::icsAddress():

ICS property	Source
LOCATION	address, falling back to location — only when the value is a string
URL	link, falling back to location when that string is a URL
GEO	coordinates (latitude / longitude)
DESCRIPTION	description

Worth stating explicitly in the docs: a URL in location currently emits both LOCATION: and URL:.

Scope

Docs only, no code changes.

• Replace both lists with a single accurate table of field → ICS property, including the fallback precedence
• Keep the computed-value note currently at line 58 — it stays the supported escape hatch for differently-named or differently-shaped fields
• Have events:download_link cross-reference that table rather than repeating it, so the two can't drift apart again

Tests

None. Docs-only change with nothing to assert, and no need to run the suite.

Branch

fix/reconcile-ics-field-docs off 6.x.

Part of #190.

[edalzell]

Done in #205