update "A note on updating" in developer docs

[phillxnet]

Although our existing developer docs is (post #410 merge) updated to reflect our most recent build method (testing branch only but soon to be master also), we have an anomaly regarding the "A note on updating" that references the move from a source build to a package build.

The anomaly arises from our prior (to #410) build advice specifying /opt/rockstor-dev, where are our packaging install directory is, and was, /opt/rockstor. Ergo a source install, when transitioned (systemd invocation overridden) to a package install would mostly likely result in no associated ".initrock" flag file in the newly referenced /opt/rockstor directory. But now that we use the same directory for both development (source) and package (rpm) we no longer have this separation. Thus when transitioning from a source install to an rpm install the .initrock flag file persists.

It is proposed that we modify this section to indicate the potential database migration failures that could result from transitioning from a source build to an rpm build. The rest of what is stated there still holds true however.