Improved JENKINS_HOME example (#3332)

After years of use I've come up with some improvements to the
`JENKINS_HOME.gitignore` example.

- Major performance improvement: On very large Jenkins installations that
  have been running for more than one year, there tends to be many builds
  (hundreds of thousands of builds).  The `builds` directory of these
  jobs contain millions of files which would cause Git to hang for
  several minutes on simple commands like `git status` and longer for
  committing changes.  `strace` was used on Git to figure out the
  performance impact and this proposed change includes the optimization.
  I also added a clear comment explaining the line's purpose.
- There's an example for how to include Jenkins encryption keys, and
  there's a disclaimer informing the user why they shouldn't but still
  giving an example.
- Comments have been reworded and slightly reformatted to be a little
  more clear.