Docs: allowUnreachableCode described backwards and with misleading default

[mattdmorgan]

Page URL: https://www.typescriptlang.org/v2/en/tsconfig#allowUnreachableCode

Issue: First sentence is "Set to false to disable warnings about unreachable code", but instead you set the value to true to disable warnings. Also, the default is described as being false, but the actual default is undefined which as of microsoft/TypeScript#24026 has different behavior: undefined gives warnings about unreachable code, while false gives errors.

The same two issues apply to the allowUnusedLabels documentation just below it.

[orta]

Great! Thanks, I've updated these docs

[mattdmorgan]

Looks like the PR addresses allowUnreachableCode; fwiw, the same applies to allowUnusedLabels.