docs(7.19): the RLS restriction on derived variables, and the startup Show log button

Two gaps found reviewing the pages against the code that is shipping:

- A derived variable cannot be named by an active row level security rule of
  any scope - mpe_derived_postedit.sas rejects it, because rules are validated
  against the physical table where a derived column does not exist. Neither the
  derived variables page nor row-level-security.md said so.

- The startup failure modal's Show log button (#356, in this release) was not
  documented anywhere. Added to troubleshooting, with why the browser request
  is what gets the log on Viya.

docs/row-level-security.md

@@ -41,6 +41,8 @@ This variable is an integer, and used to divide a complex filter into numerous s
 ### RLS_VARIABLE_NM
 This is the name of the variable against which a filter value will be applied
 
+It must be a physical column of the table. A [derived variable](/derived-variables/) cannot be named here: rules are validated against the table itself, where a derived column does not exist, so such a rule is rejected when it is submitted.
+
 ### RLS_OPERATOR
 The available operator will depend on whether the column is character or numeric.  Example values: