docs: Clean up public API descriptions across components - #2303

A lot of found during IgniteUI/igniteui-blazor#286 and just fixing in the source. Split into multiple commits for ease of review purposes.

Documentation-only pass over JSDoc descriptions. No runtime changes.

Why

Every description on a component, property, method, event, slot or CSS part is copied verbatim into custom-elements.json, the generated Storybook story metadata, and the published API docs for Angular / React / Blazor. A typo or a wrong description doesn't stay in the source — it ships to every consumer, in four framework flavours.

What changed

1. igc- tag names removed from prose (~30 components). Descriptions referred to components by tag name (`igc-carousel`, <igc-chat>, igc-textarea), which is meaningless in the framework wrapper docs. Covers component descriptions, @csspart, @slot, @fires and public members. Tag names kept where correct: @element, @example blocks, literal event names like "igc-change-theme", and internal-only members.
   Manifest scan: 76 → 21 descriptions containing igc-, all 21 remaining internal-only.

2. Descriptions that were factually wrong — see the list below.

3. Redundant phrasing. The label attribute of the control. → The label of the control. (@attr already says it's an attribute); Gets/Sets … replaced with declarative wording; booleans now read Whether … and describe the true state. Applied to all ~30 occurrences, including name on the shared form-associated mixin (so it lands on every form control) and the native attributes on igc-input / igc-file-input.

4. Typos and grammar (~15): use → user interactions, tha → the, ot → of, comma separate string → separated, DropDown → Dropdown, if it invalid → if it's invalid, and a few future → present tense fixes.

5. Missing docs added. igc-input and igc-radio had no component description at all (empty summary in Storybook). Also DateRangeDescriptor.dateRange, DatePartDeltas, IconMeta, and @returns on the expansion panel's toggle() / hide() / show().

6. Regenerated Storybook metadata via npm run cem && npm run build:meta. All generated (// region default); several stories had drifted out of sync with the source. stories/splitter.stories.ts has no region markers and was updated by hand.

7. stories/datepicker.stories.ts → date-picker.stories.ts (rename). build-stories.mjs derives the story filename from the tag name, so for igc-date-picker it looked for date-picker.stories.ts, didn't find it, and logged "No story file found for IgcDatePickerComponent, skipping." — silently, on every build. The file had not been regenerated since the tag was renamed from igc-datepicker, and had drifted badly: component: 'igc-datepicker' pointed at a tag that no longer exists (so Storybook's docs and controls binding resolved to nothing, while the story's own templates correctly used <igc-date-picker>), the component description still literally began "igc-datepicker is a feature rich component…", and required / disabled / invalid / displayFormat / locale descriptions plus the summary default quoting were all stale.

   Renaming reconnects it to the generator. Regeneration is idempotent, and drops the locale: 'en' arg default, matching combo / date-range-picker / date-time-input / calendar, none of which seed locale either.

   Note for reviewers: this changes title: 'Datepicker' → 'DatePicker', so the story's Storybook URL changes. Nothing else in the repo referenced the old filename or title. igc-date-picker was the only component affected by this filename-mismatch trap.

Corrected descriptions worth a look

These were misleading rather than just clumsy:

• calendar.hideOutsideDays — said "Whether to show the dates…". It hides them.
• carousel.hideIndicators — said "should render the indicator controls". It skips
  rendering them.
• chip igcRemove — claimed "Returns the removed chip component." The event is
  CustomEvent<boolean> emitted with no detail.
• nav-drawer-item disabled / active — described the drawer, not the item.
• date picker / date range picker open-close events — called the calendar a "dropdown";
  it's a popover.
• calendar.values — "selection is set to multiple of range" → "or range".

Preventing regressions

The rules weren't written down anywhere, so each new component reintroduced them. Updated three
skills under .github/skills/:

• create-new-component — new Documentation Conventions section (no tag names in prose, phrasing rules, tag layout, regeneration commands) plus checklist items.
• add-component-property — new Write the Description step, including the boolean-polarity trap that produced the two inverted descriptions above. Its Storybook step used to tell you to hand-edit the generated region — now says regenerate, which is what caused the drift.
• review-component-pr — checklist items, an accept/reject example, and a grep to catch leaks.

Verification

check-types, lint:biome and lint:prettier all clean. cem + build:meta idempotent — a second run produces no diff.