Add checking nested level for {use} and {extends}

Add more docs

docs/operators.md

@@ -1,22 +1,67 @@
 Operators
 =========
 
-Math
+### Math
 
-`+ - / *`
+Operators: `+ - / *`
 
-Bitwize
+```smarty
+{$a + $b * $c/$d - $e*5 + 1e3}
+```
 
-`| & << >> |= &= <<= >>=`
+### Boolean
 
-Unary
+Operators:  `|| && and or < > <= >= == === !== !=`
 
-`^ ~ - !`
+```smarty
+{if $a && $b >= 5 && $c != 3} {/if}
+```
 
-Boolean
+### Bitwize
 
-`|| && and or < > <= >= == === !== !=`
+Operators: `| & << >> |= &= <<= >>=`
 
-Ternar
+```smarty
+{if $a & 1} {var $b |= $flags} {/if}
+```
 
-`? :`
+### Unary
+
+Operators:  `^ ~ - !`
+
+```smarty
+{var $b |= $flags & ^$c}
+```
+
+### Ternar
+
+Operators: `? :`
+
+```smarty
+{var $a = true}
+{$a ? 5 : 10} {* outputs 5 *}
+{var $a = false}
+{$a ? 5 : 10} {* outputs 10 *}
+```
+
+### Variable operator
+
+Checking variable value
+```smarty
+{if $a?} {* instead of {if !empty($a)} *}
+```
+
+Checking variable existence
+```smarty
+{if $a!} {* instead of {if isset($a)} *}
+```
+
+Get default if variable is empty
+```smarty
+{$a?:"some text"} {* instead of {if empty($a) ? "some text" : $a} *}
+```
+
+Get default if variable doesn't exist
+```smarty
+{$a!:"some text"} {* instead of {if isset($a) ? $a : "some text"} *}
+```