Spec for nimble-tooltip (#595)

# Pull Request

## 🤨 Rationale

Completed spec for nimble-tooltip (#309)


## ✅ Checklist

- [x] I have updated the project documentation to reflect my changes or determined no changes are needed.

change/@ni-nimble-components-f1b0b2b1-f154-4ace-9825-91316a7e941e.json

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+{
+  "type": "none",
+  "comment": "tooltip spec (#309)",
+  "packageName": "@ni/nimble-components",
+  "email": "[email protected]",
+  "dependentChangeType": "none"
+}

packages/nimble-components/src/tooltip/tooltip.spec.md

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+# Nimble Tooltip
+
+## Overview
+
+The `nimble-tooltip` is a popup that displays information related to an element when the element receives keyboard focus or the mouse hovers over it. It typically appears after a small delay and disappears when Escape is pressed or on mouse out. The Nimble tooltip is based upon [FAST's tooltip component](https://github.com/microsoft/fast/tree/master/packages/web-components/fast-foundation/src/tooltip)
+
+The nimble-tooltip project will first be implemented as a prototype, open issues listed below will be addressed once the prototype is functional.
+
+### Background
+
+[Nimble issue #309: Tooltip](https://github.com/ni/nimble/issues/309)
+
+[Visual desgin spec](https://xd.adobe.com/view/8ce280ab-1559-4961-945c-182955c7780b-d9b1/screen/044414d7-1714-40f2-9679-2ce2c8202d1c/specs/)
+
+---
+
+## Design
+
+### API
+
+[FAST tooltip API](https://github.com/microsoft/fast/blob/de7f234ef871204fcac2b5df59433d919809341d/packages/web-components/fast-foundation/src/tooltip/tooltip.spec.md)
+
+Plan to extend API to support all cases shown in visual design XD document. Will add
+once tooltip is sucessfully implemented with one case.
+
+Plan to first implement the tooltip and let the client choose which type of tooltip (general, error, info) they want
+Additional changes to API expected in the future, but will not be included in first pass of implementation. Listed in Future Improvements and Open Issues
+
+-   _Component Name:_ `nimble-tooltip`
+-   _Properties/Attributes:_ Unchanged
+-   _Methods:_ Unchanged
+-   _Events:_ Unchanged
+-   _CSS Classes and Custom Properties that affect the component:_ None
+-   _Slots:_ Unchanged
+-   _Template:_ Unchanged
+
+### Future Improvements
+
+Attribute for switching between error and info states of tooltip
+Easier integration with other nimble components
+
+### Angular integration
+
+An Angular directive will be created for the component. The component will not have form association, so a `ControlValueAccessor` will not be created.
+
+### Blazor integration
+
+A Blazor wrapper will be created for the component.
+
+### Additional requirements
+
+-   _User interaction: Do the FAST component's behaviors match the visual design spec? When they differ, which approach is preferable and why?_
+    -   No additional requirements
+-   _Styling: Does FAST provide APIs to achieve the styling in the visual design spec?_
+    -   FAST API most likely won't be sufficient for creating extra states in spec, will be adressed later on.
+-   _Testing: Is FAST's coverage sufficient? Should we write any tests beyond Chromatic visual tests?_
+    -   Will look at options as building, testing may be difficult because only displayed on hover.
+-   _Documentation: Any requirements besides standard Storybook docs and updating the Example Client App demo?_
+    -   No additional requirements
+-   _Tooling: Any new tools, updates to tools, code generation, etc?_
+    -   No additional requirements
+-   _Accessibility: keyboard navigation/focus, form input, use with assistive technology, etc._
+    -   Aria-describedBy recommendation in storybook docs for tooltip for developers
+-   _Globalization: special RTL handling, swapping of icons/visuals, localization, etc._
+    -   No additional requirements
+-   _Performance: does the FAST component meet Nimble's performance requirements?_
+    -   No additional requirements
+-   _Security: Any requirements for security?_
+    -   No additional requirements
+
+---
+
+## Open Issues
+
+When user is using nimble tooltip and nimble components, is there an easier way to add a tooltip so they wouldn't have to anchor to an html element every time they want a tooltip?
+
+-   Update nimble controls to have tooltip show and be tooltip aware
+-   Create a tooltip in the shadow dom internally, would not require a separate tooltip element.
+-   If we don't do anything, at least provide an easy way to create unique ids.
+
+Can tooltip be found by screen reader?
+
+For testing, can we force the tooltip to be visible without hover? [Possible Ideas](https://stackoverflow.com/questions/62043424/mock-hover-state-in-storybook)
+
+Mobile tooltip is not very functional- have to click on button to show tooltip, and clicking away does not make it disappear