Clarify writes of non-supported values to WLRL fields do not throw an illegal instruction exception.

Created by: jm4rtin

In section 2.3 of the RISC-V specification it states:

Implementations are permitted but not required to raise an illegal instruction exception if an instruction attempts to write a non-supported value to a WLRL field.

This line in the documentation clarifies this implementation choice.

Signed-off-by: John Eric Martin John.Martin@emmicro-us.com