`just --list` support multi-line documentation comments

I think there are a few things that could make the list view more useful as a "usage" document for a given Justfile.

Currently, if we have a Justfile such as this:

# Cool summary of command functionality!
#
# Some extra warnings, longer documentation, and explanation 
# of what this command does.
command: 
    echo "hello"

Then we run just --list, it prints the last line of the comment above the command, which is a bit inconvenient.

Available recipes:
    command                 # of what this command does.

I think supporting multiline documentation would be valuable, where the default of --list is to show just the first line of the comment, instead of the last, and then in just --show command it would render the whole long-form comment.

It could also be useful to add a --list --full or other such flag to request the long-form documentation on the list view as well, if desired (or use the distinction between -l and --list for this, similar to some tools that use -h and --help for short/long form help test)