Update doc building instructions
Update doc building instructions

Few reasons why this change is needed:
  - the `--enable-docs` switch is easy to forget.
  - the Vagrant config doesn't even set up sphinx
  - not having the python sphinx package installed generates
    very ambiguous error messages
reveller/trafficserver,vmamidi/trafficserver,davidbz/trafficserver,clearswift/trafficserver,taoyunxing/trafficserver,bryancall/trafficserver,davidbz/trafficserver,PSUdaemon/trafficserver,PSUdaemon/trafficserver,PSUdaemon/trafficserver,persiaAziz/trafficserver,reveller/trafficserver,dyrock/trafficserver,SolidWallOfCode/trafficserver,chitianhao/trafficserver,reveller/trafficserver,SolidWallOfCode/trafficserver,taoyunxing/trafficserver,davidbz/trafficserver,reveller/trafficserver,chitianhao/trafficserver,PSUdaemon/trafficserver,bryancall/trafficserver,PSUdaemon/trafficserver,pbchou/trafficserver,rpufky/trafficserver,dyrock/trafficserver,rpufky/trafficserver,persiaAziz/trafficserver,vmamidi/trafficserver,taoyunxing/trafficserver,dyrock/trafficserver,duke8253/trafficserver,duke8253/trafficserver,reveller/trafficserver,chitianhao/trafficserver,bryancall/trafficserver,rahmalik/trafficserver,rahmalik/trafficserver,persiaAziz/trafficserver,PSUdaemon/trafficserver,persiaAziz/trafficserver,chitianhao/trafficserver,pbchou/trafficserver,persiaAziz/trafficserver,rpufky/trafficserver,PSUdaemon/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,davidbz/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,persiaAziz/trafficserver,bryancall/trafficserver,bryancall/trafficserver,taoyunxing/trafficserver,rpufky/trafficserver,pbchou/trafficserver,duke8253/trafficserver,rahmalik/trafficserver,PSUdaemon/trafficserver,vmamidi/trafficserver,clearswift/trafficserver,clearswift/trafficserver,rpufky/trafficserver,reveller/trafficserver,rahmalik/trafficserver,davidbz/trafficserver,persiaAziz/trafficserver,rpufky/trafficserver,rpufky/trafficserver,SolidWallOfCode/trafficserver,rpufky/trafficserver,clearswift/trafficserver,taoyunxing/trafficserver,chitianhao/trafficserver,vmamidi/trafficserver,PSUdaemon/trafficserver,SolidWallOfCode/trafficserver,dyrock/trafficserver,clearswift/trafficserver,pbchou/trafficserver,rahmalik/trafficserver,clearswift/trafficserver,pbchou/trafficserver,duke8253/trafficserver,clearswift/trafficserver,reveller/trafficserver,reveller/trafficserver,rahmalik/trafficserver,reveller/trafficserver,rahmalik/trafficserver,rahmalik/trafficserver,duke8253/trafficserver,clearswift/trafficserver,bryancall/trafficserver,taoyunxing/trafficserver,taoyunxing/trafficserver,SolidWallOfCode/trafficserver,dyrock/trafficserver,rpufky/trafficserver,rahmalik/trafficserver,chitianhao/trafficserver,chitianhao/trafficserver,taoyunxing/trafficserver,persiaAziz/trafficserver,duke8253/trafficserver,dyrock/trafficserver,vmamidi/trafficserver,clearswift/trafficserver,davidbz/trafficserver,taoyunxing/trafficserver,pbchou/trafficserver,dyrock/trafficserver