ESI plugin documentation updates. (#7970)
ESI plugin documentation updates. (#7970)

This makes some minor tweaks to the ESI plugin document. The only
significant change to the doc is to remove the language saying that
`--enable-experimental-plugins` is needed to configure ATS to build the
ESI plugin. That's removed because it's not true: the ESI plugin has
been upgraded from an experimental plugin to a standard plugin for a few
releases already.
SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,vmamidi/trafficserver,pbchou/trafficserver,duke8253/trafficserver,pbchou/trafficserver,duke8253/trafficserver,pbchou/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,pbchou/trafficserver,duke8253/trafficserver,pbchou/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,duke8253/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,pbchou/trafficserver