[5.2] Sync collections documentation

[paulofreitas]

Syncs Laravel 5.2 collections documentation (including Eloquent collections)

Added

• Added 4 missing methods:
  • average() (backports [5.4] Misc improvements to collections documentation #3228)
  • median() and mode() from [5.2] Add median and mode functions to collections framework#14305 (since v5.2.40; backports [5.4] Document Collection::median() and Collection::mode() methods #3223)
  • pipe() from [5.2] Add pipe function to collection framework#13899 (since v5.2.40)
• Added 15 missing methods to Eloquent collections documentation; all the listed above plus:
  • avg() (since v5.1.18)
  • combine() (since v5.2.24)
  • diffKeys() (since v5.2.15)
  • except() and only() (since v5.1.24)
  • flatMap (since v5.1.25)
  • max() and min() (since v5.1.2)
  • union() (since 5.2.27)
  • whereIn() and whereInLoose() (since v5.2.12)

Changed

• Backported all the great documentation improvements from newer versions (wording, examples, etc.) while not changing any API usage neither notes formatting (to keep consistency)
• Improved contains() and unique() methods to describe their comparison behavior

Keeping documentation synced is a bit tricky, but since many people still use older versions I believe it's worth. :)