Remove Latin terms from user docs

**Describe the bug**
https://element.io/help uses Latin in en_US/GB user-facing documentation.

It is bad practice to use Latin in English language documentation because it adds obscurity with no benefit, which can alienate users (especially those who are not native speakers). Additionally, the Latin terms are not formatted correctly (they should be italicised).

For example, search the page for "a posteriori".

**Expected behavior**
User help should use simple and clear language wherever possible.