[franka_description] Use proper title; make more searchable

[EricCousineau-TRI]

During investigation that yielded #300 #301

[rickstaa]

@EricCousineau-TRI, thank you for submitting this pull request. I believe it's a valuable addition, as it provides essential information to users about the origin of simulated inertia values – great job! 👍🏻

@Maverobot, I apologize for tagging you in multiple PRs, but considering that this pull request only modifies docstrings, I believe it can be merged seamlessly without any concerns. Your input on this would be appreciated.

[rickstaa]

@EricCousineau-TRI, would it be possible to consider changing the PR title and commit message to something like:

docs(franka_gazebo): add inertia properties reference

This adjustment would make it more evident at a glance what the PR is focused on, as the current title doesn't immediately convey its purpose.

[Maverobot]

@rickstaa No problem. :D Since I will leave Franka soon, please consider tagging someone else such as @FE-EnricoSartori so that the progress can be better tracked.