doc: mention x:h@e variant in docstring of x@e (leanprover#3073)

This was done in 1c1e6d7

[Zulip
thread](https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/Naming.20equality.20hypothesis.20in.20match.20branch/near/408016140)

src/Lean/Elab/PatternVar.lean

@@ -146,7 +146,6 @@ partial def collect (stx : Syntax) : M Syntax := withRef stx <| withFreshMacroSc
       ```
       def namedPattern := check... >> trailing_parser "@" >> optional (atomic (ident >> ":")) >> termParser
       ```
-      TODO: pattern variable for equality proof
      -/
     let id := stx[0]
     discard <| processVar id

src/Lean/Parser/Term.lean

@@ -669,7 +669,8 @@ def isIdent (stx : Syntax) : Bool :=
   checkStackTop isIdent "expected preceding identifier" >>
   checkNoWsBefore "no space before '.{'" >> ".{" >>
   sepBy1 levelParser ", " >> "}"
-/-- `x@e` matches the pattern `e` and binds its value to the identifier `x`. -/
+/-- `x@e` or `x:h@e` matches the pattern `e` and binds its value to the identifier `x`.
+If present, the identifier `h` is bound to a proof of `x = e`. -/
 @[builtin_term_parser] def namedPattern : TrailingParser := trailing_parser
   checkStackTop isIdent "expected preceding identifier" >>
   checkNoWsBefore "no space before '@'" >> "@" >>