[linter] should validate help example option place-holders

After #8083 is merged, work should be done to validate help example parameter placeholders. Parameter placeholders should be surrounded by curly braces and have CamelCase text. The first character can be a capital letter.

Placeholders like:

• <some value>
• [some-value]

Should be replaced with:

• {SomeValue}

Note that values / Placeholders like:

• somevalue
• "somevalue"
• {SomeValue} (only this must be camel-case)
• $somevar
• @some_file.txt
• $(echo foo)

Also update authoring docs with guidelines on authoring command examples.

cc/ @sptramer