Add documentation into the LangRef for parsing nested symbols.
Reviewed By: rriddle
Differential Revision: https://reviews.llvm.org/D122977
value-id ::= `%` suffix-id
suffix-id ::= (digit+ | ((letter|id-punct) (letter|id-punct|digit)*))
-symbol-ref-id ::= `@` (suffix-id | string-literal)
+symbol-ref-id ::= `@` (suffix-id | string-literal) (`::` symbol-ref-id)?
value-id-list ::= value-id (`,` value-id)*
// Uses of value, e.g. in an operand list to an operation.