docs: Updated description of ULP FSM shift operations

This commit updates the documentation for ULP FSM LSH and RSH
operations.

Closes #8831

docs/en/api-reference/system/ulp_instruction_set.rst

@@ -281,6 +281,9 @@ The detailed description of all instructions is presented below:
 **Description**
    The instruction does a logical shift to left of the source register by the number of bits from another source register or a 16-bit signed value and stores the result to the destination register.
 
+.. note::
+   Shift operations which are greater than 15 bits will have an undefined result.
+
 **Examples**::
 
   1:       LSH R1, R2, R3            // R1 = R2 << R3
@@ -316,6 +319,9 @@ The detailed description of all instructions is presented below:
 **Description**
   The instruction does a logical shift to right of a source register by the number of bits from another source register or a 16-bit signed value and stores the result to the destination register.
 
+.. note::
+   Shift operations which are greater than 15 bits will have an undefined result.
+
 **Examples**::
 
   1:        RSH R1, R2, R3              // R1 = R2 >> R3