Add prose documentation for file/directory attachments.

[grynspan]

Document how to attach a file or directory to a test.

Checklist:

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

[jgmcnutt]

Minor edits.

[jgmcnutt]

Suggested change

	its file URL. The testing library must read or map the file before attaching it
	to your test, and those operations can fail, so you must explicitly create an
	instance of ``Attachment`` before you record it.
	its file URL. The testing library needs to read or map the file before attaching it
	to your test, and those operations can fail, so you need to explicitly create an
	instance of ``Attachment`` before you record it.