Add TraditionalSwitchExpression.md.

PiperOrigin-RevId: 654138846

docs/bugpattern/TraditionalSwitchExpression.md

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+The newer arrow (`->`) syntax for switches is preferred to the older colon (`:`)
+syntax. The main reason for continuing to use the colon syntax in switch
+*statements* is that it allows fall-through from one statement group to the
+next. But in a switch *expression*, fall-through would only be useful if the
+code that falls through has side effects. Burying side effects inside a switch
+expression makes code hard to understand.