improves some form component descriptions in docs (#1923)

Author: mperrotti

docs/content/CheckboxGroup.mdx

@@ -1,6 +1,6 @@
 ---
 title: CheckboxGroup
-description: A `CheckboxGroup` is used to render a set of checkboxes to let users select one or more options
+description: Renders a set of checkboxes to let users make one or more selections from a short list of options
 status: Alpha
 source: https://github.com/primer/react/blob/main/src/CheckboxGroup/CheckboxGroup.tsx
 storybook: '/react/storybook/?path=/story/forms-checkboxgroup-examples--basic'

docs/content/FormControl.mdx

@@ -2,7 +2,7 @@
 componentId: form_control
 title: FormControl
 status: Alpha
-description: The FormControl component is used to render a labelled text input and, optionally, associated validation text and hint text.
+description: Renders a labelled input and, optionally, associated validation text and/or hint text.
 source: https://github.com/primer/react/blob/main/src/FormControl/FormControl.tsx
 storybook: '/react/storybook?path=/story/forms-inputfield--text-input-field'
 ---

docs/content/RadioGroup.mdx

@@ -1,6 +1,6 @@
 ---
 title: RadioGroup
-description: A `RadioGroup` is used to render a set of radio inputs to let users select a single option
+description: Renders a set of radio inputs to let users make a single selection from a short list of options
 status: Alpha
 source: https://github.com/primer/react/blob/main/src/RadioGroup/RadioGroup.tsx
 storybook: '/react/storybook/?path=/story/forms-radiogroup-examples--basic'