[Instruments] Correct/Update TestPlan and Readme for DoD feature / linst

[christinerogers]

#4929 introduced a visible bug (fix #7397) in every Linst instrument form which wasn't caught during 2 rounds of release testing (23 spring 2020 and mid-release fall 2020)
Would have been caught if we tested a sample Linst instrument at every release and/or if #4929 updated relevant documentation.

Documentation to be updated:

• Instrument module Readme : Configuration section
• Instrument module test plan : at least a basic form save should be tested on a Linst instrument, so that we have a chance to catch issues.