Align pattern destructuring with rest of doc

Previously, `patterns.destructure.placeholder` referred to the `_` in a
destructuring pattern as "placeholder" and `..` as "wildcard". This
directly contradicted the definition of struct patterns and tuple
(struct) patterns, which (indirectly) define `_` as a wildcard pattern
and `..` as a structure pattern et cetera (for structs, enums, and union
values) or rest pattern (for tuples, tuple structs, slices, and arrays).

Update the paragraph to use "wildcard pattern", "et cetera", and "rest
pattern" instead.  Moreover, add links to their respective
sections/statements.

Don't update the reference `r[patterns.destructure.placeholder]`, as
changes might break (third-party) links to the updated paragraph.

Author: archer-321

src/patterns.md

@@ -93,7 +93,7 @@ Destructuring breaks up a value into its component pieces.
 The syntax used is almost the same as when creating such values.
 
 r[patterns.destructure.placeholder]
-In a pattern whose [scrutinee] expression has a `struct`, `enum` or `tuple` type, a placeholder (`_`) stands in for a *single* data field, whereas a wildcard `..`  stands in for *all* the remaining fields of a particular variant.
+In a pattern whose [scrutinee] expression has a `struct`, `enum` or `tuple` type, a [wildcard pattern](#wildcard-pattern) (`_`) stands in for a *single* data field, whereas an [et cetera](#grammar-StructPatternEtCetera) or [rest pattern](#rest-patterns) (`..`) stands in for *all* the remaining fields of a particular variant.
 
 r[patterns.destructure.named-field-shorthand]
 When destructuring a data structure with named (but not numbered) fields, it is allowed to write `fieldname` as a shorthand for `fieldname: fieldname`.