Update Snowplow documentation

What does this MR do?

As @cwoolley-gitlab pointed out (!47589 (comment 449717556)) there is confusing detail about instrumenting frontend code with snowplow, that may cause events to not be collected

I was confused, because I was looking at and copying this example, which doesn't require wrapping in html_options.

But, that's only because of the behavior of link_to when it is passed a block - it accepts the second argument as html_options hash if passed a block.

The custom GitLab nav_link helper does not have this behavior, you have to explicitly pass the named html_options argument.

This MR aims to state difference between link_to and nav_link method, so the problem can be avoided

Related issues

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