docs(config): optional VAR_LABEL

docs/tables/mpe_derived.md

@@ -21,5 +21,6 @@ The table is itself registered in [MPE_TABLES](/tables/mpe_tables/), so it is ma
  - `VAR_TYPE char(1)`: `C` for character, `N` for numeric
  - `VAR_LEN num`: Character length, or numeric length (1-8)
  - `VAR_FMT char(32)`: Optional SAS format, eg `DATE9.`
+ - `VAR_LABEL char(256)`: Optional label; defaults to `<VAR_NM>=<expression>`
  - `VAR_LOGIC char(4000)`: The SAS expression that produces the value
  - `ACTIVE num`: Set to 1 to apply the variable