MSVC /Wall is not unusable because of standard library headers

The current wording reads "`/Wall` - Also warns on files included from the standard library, so it's not very useful and creates too many extra warnings." – this reason does not seem to hold, because `/external` (i.e. `/external:anglebrackets`) exists and is effective in preventing `/Wall` warnings in the standard headers too.

A better reason to avoid `/Wall` would be a large number of informational diagnostics (i.e. https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4711?view=msvc-170) - but then again, these can be disabled individually.

審查 issue 中描述的 `/Wall` 指引，從現有措辭和連結的 MSVC 警告文件開始。驗證 `/external` 如何影響標準函式庫警告，以及 C4711 等資訊性診斷是否才是更準確的關注點。完成的標準是，文件不再給出避免使用 `/Wall` 的錯誤理由，並準確反映可用的警告控制方式。

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