[SR-Users] $ru vs. $fu as r/w pseudo variables

[Juha Heinanen]

Perhaps it would best to explicitly tell on Pseudo-Variables Wiki page,
which R/W pseudo "variables" are not really variables in the sense that
setting of the value does not change its value.

Are there others than these three: $fd, $fu, $fU.  If not, I can add a
note to their entries.

-- Juha