doc: remove em dashes

Our documentation uses em dashes inconsistently. They are treated inconsistently typographically too. (For example, they are sometimes surrounded by spaces and sometimes not.) They are also often confused with ordinary hyphens such as in the CHANGELOG, where they are inadvertently mixed together in a single list. The difference is not obvious in the raw markdown but is very noticeable when rendered, appearing to be a typographical error (which it in fact is).

The em dash is never needed. There are always alternatives. Remove em dashes entirely.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines