[doc/icache] Document the err_plus2_o signal
[doc/icache] Document the err_plus2_o signal

Add a few sentences to describe the behaviour/meaning of the err_plus2_o
signal, and how it is used by the core.

Signed-off-by: Tom Roberts <f3b1a53ee1cbeb35a47033dea932030093853292@lowrisc.org>

AmbiML/ibex,lowRISC/ibex,AmbiML/ibex,lowRISC/ibex,AmbiML/ibex,lowRISC/ibex,lowRISC/ibex,AmbiML/ibex