Update match-guards.md (#233)

* Update match-guards.md

Adding more information how match guards are different from simply using "if" inside of the match case, after the case has matched.

* Be consistent about naming of match arms.

Co-authored-by: Andrew Walbran <qwandor@google.com>

Author: igor-petruk

src/pattern-matching/match-guards.md

@@ -6,3 +6,14 @@ expression which will be executed if the pattern matches:
 ```rust,editable
 {{#include ../../third_party/rust-by-example/match-guards.rs}}
 ```
+
+<details>
+
+Match guards as a separate syntax feature are important and necessary. They are not
+the same as separate `if` expression inside of the match arm.
+  
+An `if` expression inside of the branch block (after `=>`) happens after the match arm
+is selected. Failing the `if` condition inside of that block won't result in other arms
+of the original `match` expression being considered.
+  
+</details>