Help: Add `.. versionadded` directives to commands documentation

Here's the first batch of manually inserted .. versionadded directives. All command docs in Help/command/* have been covered.

While most of the time simply adding .. versionadded to the relevant paragraph is enough, a few cases required paragraph splitting, rewording and reordering of help text. To some degree it's a matter of style and opinion. I've documented my "style guide for version markers" in the commit message.

Many of the changes involve indenting paragraphs to put them under .. versionadded, I didn't re-wrap to 80 columns to make reviewing easier.

Issue: #19715