docs(ngShowHide): improve clarity

Closes #4099
docs(guide/e2e-testing): Fix typo

Closes #4100
docs($http): fix adding default header to get request example

Initially, `$httpProvider.defaults.headers.get` is `undefined`, so
`$httpProvider.defaults.headers.get['My-Header']='value'` will throw an
error.

Closes #4101
docs(guide/$location): clarify $location service role

Clean up confusing use of the term URL to refer to $location as well as 'URL in the browser'.

Closes #4103