Add documentation to Parallelization.md describing how the .serialized trait can be applied to a test suite across different files.

[gmedori]

Add documentation to Parallelization.md describing how the .serialized trait can be applied to a test suite across different files.

Resolves #686

Motivation:

After a discussion in #683, we decided it would be helpful for folks to see an example of how you can use the .serialized trait to split your serialized tests across multiple files.

Modifications:

• Added a section to Parallelization.md to describe the .serialized trait's behavior when split across multiple files
• Update the title of another section in the same file to adhere to the style guide

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[gmedori]

Screenshot of how the docs look with my latest changes: