Remove erroneous "Notes" from SUB and SUBI instructions

[ironcev]

Description

This PR fixes the documentation of the SUB and SUBI instructions and makes it follow the same pattern as the ADD and ADDI instructions.

The documentation was wrongly using the term "overflow" in the "Notes" of those two instructions, instead of "underflow" (as used in the longer explanation below the instruction).

The PR simply removes the erroneous description from the "Notes" and leaves them empty, as in ADD and ADDI.

Before requesting review

• I have reviewed the code myself

[Dentosal]

Technically "integer underflow" is still an overflow, i.e. operation resulting a value outside the target domain. That word is widely used to describe this situation, for example in the llvm docs The note is not incorrect. That said, if it's unclear to you it could be improved.

[Dentosal]

Parity with add/addi is a good thing.