Add match expression page

[Girgias]

This is a W.I.P. any helped appreciated, especially @iluuu1994 for reviewing.

I'm struggling to describe some of the features and how to go about it, some of the terminology is copied from the Rust documentation: https://doc.rust-lang.org/reference/expressions/match-expr.html

[Crell]

Still missing, IMO:

• Description of what is thrown if no expression matches.
• You may want to explicitly call out the need for the closing semicolon. I don't know what conventions are on that, but it's not obvious for people used to } not needing a semicolon.
• Explicitly state how compound arms work. It's implied, but not stated explicitly. Let's be explicit.

Recommended: A comparison with ternaries, not just switch. I've been saying since day 1 that match is more akin to a multi-armed ternary than it is to a switch statement. 😄

Optional: Note that if more complex checks than === are needed, match (true) {} can handle those cases.

[Girgias]

Still missing, IMO:

* Explicitly state how compound arms work.  It's implied, but not stated explicitly.  Let's be explicit.

Don't really know how to write this up in all honesty.

Recommended: A comparison with ternaries, not just switch. I've been saying since day 1 that match is more akin to a multi-armed ternary than it is to a switch statement. 😄

I made a mention about it but I don't imagine that's what you want.

[Crell]

Suggested change

	it will evaluate to a value much like ternary expressions
	it will evaluate to a value much like ternary expressions.

[Crell]

Suggested change

	<literal>Match</literal> arms do not fallthrough compared to switch cases.
	<literal>Match</literal> arms do not fall-through to later cases the way <literal>switch</literal> statements do.

[Girgias]

Merged as http://svn.php.net/viewvc?view=revision&revision=351499