Add VAL documentation

Also fix links in STR doc

Author: boriel

docs/str.md

@@ -28,5 +28,4 @@ PRINT STR(a + 1)
 
 * [CHR](chr.md)
 * [CODE](code.md)  
-* [STR](str.md)
 * [VAL](chr.md)

docs/val.md

@@ -0,0 +1,38 @@
+#VAL
+
+##Syntax
+
+
+```
+VAL(<string value>)
+```
+
+##Description
+
+Converts the given numeric string value into its numeric value. It's the opposite of [STR](str.md).
+If the string can be converted into a number, [PEEK](peek.md) 23610 (ROM ERR_NR variable) will return 255 (_Ok_).
+
+On the other side, if expression cannot be parsed (i.e. it's not a valid number expression), 0 will be returned,
+and [PEEK](peek.md) 23610 (ROM ERR_NR variable) will return 9 (_Invalid Argument_).
+
+Returned value type is [Float](types.md#float).
+
+##Examples
+
+```
+REM Convert numeric expression to value
+LET a$ = "-55.3e-1"
+PRINT "Value of "; a$; " is "; VAL(a$)
+LET a$ = "aaa"
+PRINT "Numeric value of "; a$; " is "; VAL(a$): REM prints 0
+```
+
+##Remarks
+
+* This function is 100% Sinclair BASIC Compatible
+
+##See Also
+
+* [CHR](chr.md)
+* [CODE](code.md)  
+* [STR](str.md)