Add info about global variables limits in FunC (#1195)

* Update global_variables.md

* Update global_variables.md

Author: memearchivarius

docs/v3/documentation/smart-contracts/func/docs/global_variables.md

@@ -25,7 +25,7 @@ int main() {
 
 In this example, the global variable `op` is assigned the addition operator `_+_`. The program then verifies the associativity of addition using three sample integers: 2, 3, and 9.
 
-Under the hood, global variables in FunC are stored in the `c7` control register of the TVM.
+Under the hood, global variables in FunC are stored in the `c7` control register of the TVM, with a maximum limit of 31 variables.
 
 
 In FunC, you can _omit the type_ of global variable.