Make placeholder names more explicit

[davidwrede]

People may get stuck putting in the literal text (or angle brackets) from placeholder values. Either a note or clarifying text should be added when using placeholders.

[kagarmoe]

Probably multiple cards.

• What standard should we follow for placeholder values? (Check practice in MS Docs, Google Docs styleguides)
• Write a style guide entry
• Plan for implementation (probably one card for each repo)
• Implementation
• Write a Vale linter to check conformity

[IanMadd]

Food for thought: https://developers.google.com/style/placeholders