Fix wording where "unspecified" was used for defined behavior (#586)

src/fuel-vm/instruction-set.md

@@ -120,14 +120,14 @@
 
 ## Reading Guide
 
-This page provides a description of all instructions for the FuelVM. Encoding is read as a sequence of one 8-bit value (the opcode identifier) followed by four 6-bit values (the register identifiers or immediate value). A single `i` indicates a 6-bit immediate value, `i i` indicates a 12-bit immediate value, `i i i` indicates an 18-bit immediate value, and `i i i i` indicates a 24-bit immediate value. All immediate values are interpreted as big-endian unsigned integers.
+This page provides a description of all instructions for the FuelVM. Encoding is read as a sequence of one 8-bit value (the opcode identifier) followed by four 6-bit values (the register identifiers or immediate value). A single `i` indicates a 6-bit immediate value, `i i` indicates a 12-bit immediate value, `i i i` indicates an 18-bit immediate value, and `i i i i` indicates a 24-bit immediate value. All immediate values are interpreted as big-endian unsigned integers. If the instruction would be shorter than the full 32 bits, the remaining part is reserved and must be zero.
 
 - The syntax `MEM[x, y]` used in this page means the memory range starting at byte `x`, of length `y` bytes.
 - The syntax `STATE[x, y]` used in this page means the sequence of storage slots starting at key `x` and spanning `y` bytes.
 
 ### Panics
 
-Some instructions may _panic_, i.e. enter an unrecoverable state. Additionally, attempting to execute an instruction not in this list causes a panic and consumes no gas. Instructions with unspecified (reserved) part having a non-zero value will likewise panic.  How a panic is handled depends on [context](./index.md#contexts):
+Some instructions may _panic_, i.e. enter an unrecoverable state. Additionally, attempting to execute an instruction not in this list causes a panic and consumes no gas. Instructions with reserved part having a non-zero value will likewise panic.  How a panic is handled depends on [context](./index.md#contexts):
 
 - In a predicate context, cease VM execution and return `false`.
 - In other contexts, revert (described below).