_content/tour: document no newline before else

[crisman]

Many users of the Go Tour trip over using else after a newline which generates a syntax error that is hard to understand unless the reader is primed to think about a newline before the else. "tour/flowcontrol/7" ("If and else") the slide that first talks about else seems the right place to update.

Add statement about required location of else after if block, provide an invalid else code example mirroring the slide's program, and provide links to the language spec for details.

Fixes golang/tour#1481 Fixes golang/tour#1427 Fixes golang/tour#1062 Fixes golang/tour#442

[gopherbot]

This PR (HEAD: c13e7cf91223cdd80d1fd39073ddf90c94322531) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/website/+/494816 to see it.

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