[5.4] Misc improvements to collections documentation

[paulofreitas]

I did a thorough review of collections's documentation and ended up with a few minor corrections and additions:

• Added missing average() alias method (synced Eloquent collections documentation)
• Added missing containsStrict() anchor (sorry to have forgotten this!)
• Added missing cross-reference links to contains(), containsStrict(), slice(), unique() and uniqueStrict() methods
• Added missing explanations about "loose" comparisons to where(), whereIn() and whereNotIn() methods
• Minor change to merge() method to state that it also accepts another collections
• Minor change to toJson() method to better describe the returned value
• Minor formatting fixes, mostly on code snippets

Just to make Laravel's documentation even more awesome. ❤️

[taylorotwell]

Thanks 👍