[PSR-7] Clarify superglobals/query parameters

- Updated docblocks for query, cookie, and file parameters to indicate that the
  data sources MAY come from superglobals, or MAY come from other sources, but
  MUST mimic the structure for the corresponding superglobal.
- Clarified that when injecting query parameters manually from values parsed
  from the URI, the rules that PHP's parse_str() follows MUST be followed.
[PSR-7] Updated IncomingRequestInterface param/return values

- Return values are stipulated to be arrays in all cases.
- Body parameters stipulate only arrays as well, not objects, for consistent
  runtime behavior between implementations.
- `s/PathParams/Attributes/`, per @simensen
Merge pull request #354 from weierophinney/hotfix/superglobals

[PSR-7] Clarify superglobal usage