[skip-ci] Expression Lifecycle Docs - Understanding the Syntax and Expressions Always Start with a Function

[shaunmcgough]

Summary

Add Understanding the Syntax and Expressions Always Start with a Function sections to expression docs.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

- [ ] This was checked for cross-browser compatibility, including a check against IE11

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
  - [ ] Unit or functional tests were updated or added to match the most common scenarios
  - [ ] This was checked for keyboard-only and screenreader accessibility

For maintainers

- [ ] This was checked for breaking API changes and was labeled appropriately
- [ ] This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/kibana-canvas (Team:Canvas)

[shaunmcgough]

This should be related to #51494 and #23032

[timductive]

Hey @shaunmcgough maybe create the PR against my expression-docs feature branch so we can merge them all into a branch, clean up any issues and then merge to master.