docs(directive): link to directive registration api function

Originally, this issue was regarding documenting `restrict: 'CM'` in the directive guide, but it
was pointed out that the restrict documentation is covered in the $compile documentation. Because of
this, a link was simply added to the $compile documentation.

However, the wording suggests that it's actually linking to the directive registration function, in
$compileProvider, so the docs will link there instead. There is a link only a paragraph below to the
$compile documentation, so this does not hurt.

Closes #5516