srd: add TODO comment on the SIGROKDECODE_DIR env var's motivation

srd: add TODO comment on the SIGROKDECODE_DIR env var's motivation

A comment in the srd.c implementation suggests that the environment
variable SIGROKDECODE_DIR would serve debugging purposes and would
override other incarnations of available decoders. I disagree with
either of these interpretations. Add a TODO comment, the phrase may
need an update or rework.