It is not clear how to install sources for the running kernel

[vvhk]

On Ubuntu 18.04, with `apt-get source <packagename>` being the de facto way of getting (src-deb) sources for a binary package, it is not so with the linux kernel.

`apt-get source linux-image-$(uname -r)` -- leads to "linux-signed" which is not the kernel src deb
`apt-get source linux-image-generic` -- leads to "linux-meta" which is, again, not it
`apt-get source linux` -- obtains the correct src deb but that's not transparent or obvious to new users and a deviation from the usual standard of using `apt-get source`.

Prior discussion and background on #ubuntu-devel:

* https://irclogs.ubuntu.com/2019/05/24/%23ubuntu-devel.html#t10:55

[Robie Basak (racb)]

Thank you for taking the time to report this bug and helping to make Ubuntu better.

Based on the IRC discussion, I think there's consensus amongst Ubuntu developers that we can do something (even if documentation) to make it easier to find the right sources, so this bug is valid.

[Robie Basak (racb)]

(however, I don't think there's any obvious package to assign it to; a solution, once decided, won't necessarily be in the linux package)