Revise description of simple_designation in Declaration pattern

[Nigel-Ecma]

The quoted comment is part of a thread, previous comments should be considered also.

---

This suggestion will fix the Word converter (the discard link):

Given a pattern input value ([§11.1](patterns.md#111-general)) *e*, if the *simple_designation* is *discard_designation*, it denotes a discard ([§9.2.9.2](variables.md#9292-discards)), and the value of *e* is not bound to anything. (Although a declared variable with the name `_` may be in scope at that point, that named variable is not seen in this context.) Otherwise, if the *simple_designation* is *single_variable_designation*, a local variable ([§9.2.9](variables.md#929-local-variables)) of the given type named by the given identifier is introduced. That local variable is assigned the value of the pattern input value when the pattern *matches* the value.  

Originally posted by @BillWagner in #873 (comment)