fix(docs): component descriptions

[kuzhelov]

Goal

This PR aims to ensure that for each component there are the following pieces provided

• concise description that reflects component's purpose
• brief accessibility description

Visual aspects of how this data is presented by docs are left outside of this PR, as its main focus is to refine, organise and make consistent the actual content (component description) that should be rendered.

Unify component names' formatting in descriptions

Previously there were no consistency, and both these approaches were used for Stardust components (based on previous component descriptions):

A Alert displays information that explains nearby content.

A button indicates a possible user action.

This PR unifies the approach for all components, by favouring the uppercased way for component references. This is done for the following reasons:

• to ensure close relationship to other Stardust component is conveyed by the description, and it is not that just some general noun/term used.
• at times it even makes an important semantic distinction - just compare:

A ChatItem is container for Chat item's content.

A chat item is container for chat item's content.

This is not the only example - similar semantic differences could be seen for the following components:

• Input
• TollbarMenuDivider
• Embed
• ToolbarDivider

References used

• WAI-ARIA 1.1
• Reakit
• Reach UI
• Material UI
• React Bootstrap
• Blueprint
• Ant Design
• Semantic UI React
• Office UI Fabric

[codecov]

Codecov Report

Merging #1629 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1629   +/-   ##
=======================================
  Coverage   71.28%   71.28%           
=======================================
  Files         852      852           
  Lines        7045     7045           
  Branches     2029     2029           
=======================================
  Hits         5022     5022           
  Misses       2017     2017           
  Partials        6        6

Impacted Files	Coverage Δ
packages/react/src/components/Header/Header.tsx	100% <ø> (ø)	⬆️
packages/react/src/components/Label/Label.tsx	95.23% <ø> (ø)	⬆️
packages/react/src/components/Button/Button.tsx	80% <ø> (ø)	⬆️
packages/react/src/components/List/ListItem.tsx	100% <ø> (ø)	⬆️
packages/react/src/components/Divider/Divider.tsx	100% <ø> (ø)	⬆️
...s/react/src/components/Toolbar/ToolbarMenuItem.tsx	68.96% <ø> (ø)	⬆️
packages/react/src/components/Video/Video.tsx	76.92% <ø> (ø)	⬆️
packages/react/src/components/Flex/FlexItem.tsx	78.57% <ø> (ø)	⬆️
...ackages/react/src/components/Reaction/Reaction.tsx	81.81% <ø> (ø)	⬆️
packages/react/src/components/Menu/MenuDivider.tsx	100% <ø> (ø)	⬆️
... and 57 more

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