Apply suggestions from code review

Co-authored-by: Travis Cross <tc@traviscross.com>

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -774,7 +774,7 @@ impl<'a> State<'a> {
                 }
             }
             ast::ExprKind::Yield(e, YieldKind::Postfix) => {
-                // it's not possible to have a postfix yield with no expression.
+                // It's not possible to have a postfix yield with no expression.
                 let e = e.as_ref().unwrap();
                 self.print_expr_cond_paren(
                     e,

compiler/rustc_parse/src/parser/expr.rs

@@ -1311,7 +1311,7 @@ impl<'a> Parser<'a> {
             return self.parse_match_block(lo, match_span, self_arg, MatchKind::Postfix);
         }
 
-        // Post-fix yield
+        // Parse a postfix `yield`.
         if self.eat_keyword(exp!(Yield)) {
             let yield_span = self.prev_token.span;
             self.psess.gated_spans.gate(sym::yield_expr, yield_span);