XLALFindFiles(): add documentation

Description

Add documentation to XLALFindFiles(). Looks like this:

Closes #335 (closed)

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

Review Status

N/A