Add upgrade guide for 2.19.4

[daniel-beck]

Not sure whether some of these are relevant enough.

@olivergondza @oleg-nenashev WDYT?

[oleg-nenashev]

Also worth mentioning https://issues.jenkins-ci.org/browse/JENKINS-38721 and https://issues.jenkins-ci.org/browse/JENKINS-37282 (regression in 2.14, which has been fixed only in 2.19.4)

[daniel-beck]

We do have a changelog already. The idea is to point out not just bug fixes all over here, but add information relevant to users specifically when upgrading.

• The problem with JNLP agent protocols not storing (or loading) the config -- point out that their previous config changes may be ineffective.
• slaveAgentPort is important both because it introduced a new feature in a .4, and because it may now unexpectedly be effective despite being set long ago (e.g. when spinning up instances by script).
• Text boxes -- at least on OS X, horizontal scrolling doesn't show very obviously in some configurations, so point this out. Also, no need for the perhaps automatically applied line length workaround anymore.

So there's clearly some use in pointing these out for administrators doing an upgrade. But I don't see any extra information need beyond changelog for those two bugs.

Even the last two I listed, which I'm not sure about as I wrote in the original comment, are of the "not obvious brokenness but you may be able to unlearn a learned behavior you applied as workaround" type -- similar to when I pointed out that people can now use the back button in the browser a few releases back.

[oleg-nenashev]

Agreed

[daniel-beck]

Can always be optimized later, merging. Thanks @oleg-nenashev