docs: Update multiline usage override rules
docs: Update multiline usage override rules

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

clap-rs/clap,clap-rs/clap,clap-rs/clap