fix: add hint for missing document global in terminal error

[bartlomieju]

This came up on Discord as a question so I thought it's worth adding a hint for this
as it might be a common pitfall.

[bartlomieju]

DO NOT LAND.

It's not great if document is used in testing code:

Deno.test("document query selector", () => {
  document.querySelector("div");
});

running 1 test from ./tests/specs/run/document/document_test.js
document query selector ... FAILED (1ms)

 ERRORS

document query selector => ./tests/specs/run/document/document_test.js:1:6
error: ReferenceError: document is not defined
  document.querySelector("div");
  ^
    at file:///Users/ib/dev/deno/tests/specs/run/document/document_test.js:2:3

 FAILURES

document query selector => ./tests/specs/run/document/document_test.js:1:6

FAILED | 0 passed | 1 failed (6ms)

error: Test failed

It should be enhancing terminal errors in deno test too.

[EGAMAGZ]

It will also grate to add this in the documentation related to testing, the need to add a library like happy-dom, deno_dom, linkedom or JSDom to polyfill the absence of dom to this type of test done with libraries like @testing-library/preact and frontend like preact (even include an example if it is possible)

[dsherret]

LGTM