add warnings for using render methods when you want string output

[mlrawlings]

Description

Deprecate renderSync(data) and render(data, callback) in their current forms. Use renderToString instead as v4 will return a RenderResult instead of a string.

Motivation and Context

With this notice in place, there shouldn't be any breaking changes when moving from v3 to v4 that don't already warn you in the current version.

Checklist:

• My code follows the code style of this project.
• I have updated/added documentation affected by my changes.
• I have read the CONTRIBUTING document.
• No new tests, changes already covered by existing tests.
• All new and existing tests passed.

[coveralls]

Coverage increased (+0.03%) to 84.019% when pulling 99e7873 on v3-render-to-string into 91c215c on v3.