docs(ngIf): add warning regarding falsy values

[mickeywu]

Add warning message to ngIf's documentation regarding what values are considered falsy (similar to ngHide / ngShow).

[mary-poppins]

Thanks for the PR! Please check the items below to help us merge this faster. See the contributing docs for more information.

• Uses the issue template (#7293)

If you need to make changes to your pull request, you can update the commit with git commit --amend.
Then, update the pull request with git push -f.

Thanks again for your help!

[lgalfaso]

@mickeywu if this is for the 1.2.x branch, then it looks ok. For 1.3, I hope that we end up merging #4005

[mickeywu]

@lgalfaso Good point, I'll submit this against 1.2.x instead.