Updated size limit info for 'file' parameter to reflect reality.

Contrary to notes in the novaclient source, exceeding 2KB combined
size results in novaclient Overlimit error:

  novaclient.exceptions.OverLimit: Personality file content too long (HTTP 413)
Merge pull request #334 from justinclift/limit_size_doc_fix

Updated size limit info for 'file' parameter to reflect reality.