docs: documented 'string_is_char_alpha'

.wiki/Kernel-documentation.md

@@ -1389,6 +1389,9 @@ Converts the boolean in **_value_** to an alphanumeric string.
 The representation is `true` or `false`.
 This string will be written into **_string_**.
 
+#### `string_is_char_alpha(chr)` - function (bool)
+Returns whether the char (**_chr_**) is alphanumeric or not.
+
 ### symbol.h
 #### `symbol_type_E` - enum
   - **Function**