docs: Update multiline usage override rules

[jpgrayson]

The styling of usage has changed in #4188 such that the usage string is on the same line as the "Usage:" title. Previously, the usage went on the line below the title. As such, the rules have changed for how to make a multiline usage format correctly.

These updated rules achieve the following output for the documented example:

Usage: myapp -X [-a] [-b] <file>
       myapp -Y [-c] <file1> <file2>
       myapp -Z [-d|-e]

Relates-to: #4132

[epage]

Thanks! Sorry that I had missed this when making the change!