docs: Fix \bar documentation by avoiding global redefinition (#3804)

* `\bar` examples weren't rendering correctly because of `\gdef\bar`
  example when illustrating `\gdef` and `\xdef`.
* Improve `\bar` name to `\sqr` when defining squaring
  (not necessary for fix, but improved naming).
* Fix `\def` scoping test that originally seemed related.

Fixes #3803

test/katex-spec.js

@@ -3471,7 +3471,7 @@ describe("A macro expander", function() {
 
     it("\\def changes settings.macros with globalGroup", () => {
         const macros = {};
-        expect`\gdef\foo{1}`.toParse(new Settings({macros, globalGroup: true}));
+        expect`\def\foo{1}`.toParse(new Settings({macros, globalGroup: true}));
         expect(macros["\\foo"]).toBeTruthy();
     });