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/derived-variables.md

@@ -120,6 +120,7 @@ Derived columns themselves are an EDIT screen feature: the VIEW screen and the d
  - **Dependencies declared.** `VAR_DEPS` lists every input the expression uses - physical columns and derived variables alike. Statements are generated in dependency order, so no sequence number is maintained, and the declaration is checked in both directions: every reference must be declared, and every declared name must be referenced and must exist. Circular references are rejected.
  - **No collisions.** `VAR_NM` must not match an existing column of the target table, or `_____DELETE__THIS__RECORD_____`.
  - **Read-only.** Derived columns cannot be edited in the grid, and any value submitted for them is discarded before the approval package is created.
+ - **Not for row level security.** A derived variable cannot be named by an active [row level security](/row-level-security/) rule, of any scope. RLS rules are validated against the physical table, where a derived column does not exist, so a rule naming one is rejected when it is submitted.
  - **Recomputed on every read.** The value always reflects the current data and the current definition, and there is no history of previous derived values.
 
 A configuration error - a bad name, an invalid type or length, a name collision, or a reference to a derived variable that is not declared in `VAR_DEPS` - stops the EDIT screen with a message naming the variable. A syntax error or an unknown column in the expression itself is caught when the view is compiled, and stops the screen with a message naming the table.