Fixes #35203

1. Improve the message for unused value assignment diagnostic to make it clear that we are flagging an unnecessary assignment, not an unused variable
2. Add descriptions for unused value assignment and unused parameter diagnostics to allow users to specify special discard symbol names that are respected by the analyzer. There are various cases where user wants to retain unused parameters (signature compat, documentation) and unnecessary assignments to locals (ease of debugging), but do not want to blanket suppress these diagnostics.