Update guidance about continuation prompts

[daobrien]

https://stylepedia.net/style/6.0/#documenting-command-syntax > 3.4.4.1. Documenting Multiple or Long Commands

This section describes how to document long and multiple commands, and the use
of continuation markers and PS2 prompts. This advice has recently changed and we
need to make sure this section aligns with what's in our local guide.

See issue #185 Update guidance about continuation prompts in the AsciiDoc
guide.