Team description field should suggest including participation instructions

Martin Pool reported that people were applying to join the Bazaar Developers team <https://launchpad.net/~bzr> without being Bazaar developers. This could have been (and has been) fixed by changing the team description, but this isn't as obvious as it could be.

The caption for the team description field currently says only "Use plain text; URLs will be linkified". This could be tweaked to solve the above problem. Compare with the project description field, which now begins with the sentence "Include information on how to get involved with development" (inspired by bug 76016).