docs: internals += def'n for "backend", links to couple of mail threads
docs: internals += def'n for "backend", links to couple of mail threads

We keep saying "backend" on the pants-devel mailing list. Maybe we should define it somewhere.

Reviewed at https://rbcommons.com/s/twitter/r/449/

slyphon/pants,baroquebobcat/pants,dgomez10/xanon,sameerparekh/pants,jsirois/pants,kwlzn/pants,TansyArron/pants,gmalmquist/pants,15Dkatz/pants,pgroudas/pants,sameerparekh/pants,tdyas/pants,manasapte/pants,di0spyr0s/pants,jessrosenfield/pants,wisechengyi/pants,landism/pants,jtrobec/pants,areitz/pants,areitz/pants,baroquebobcat/pants,dgomez10/xanon,fkorotkov/pants,fkorotkov/pants,TansyArron/pants,megaserg/pants,scode/pants,square/pants,dturner-tw/pants,TansyArron/pants,landism/pants,dgomez10/xanon,mateor/pants,sameerparekh/pants,pombredanne/pants,square/pants,digwanderlust/pants,sid-kap/pants,dbentley/pants,fkorotkov/pants,gmalmquist/pants,di0spyr0s/pants,15Dkatz/pants,jessrosenfield/pants,tejal29/pants,dbentley/pants,ity/pants,gmalmquist/pants,benjyw/pants,slyphon/pants,pgroudas/pants,mateor/pants,jessrosenfield/pants,sameerparekh/pants,dgomez10/xanon,wisechengyi/pants,gmalmquist/pants,tdyas/pants,slyphon/pants,UnrememberMe/pants,dgomez10/xanon,square/pants,mateor/pants,benjyw/pants,wisechengyi/pants,peiyuwang/pants,di0spyr0s/pants,qma/pants,dbentley/pants,dgomez10/xanon,foursquare/pants,foursquare/pants,cevaris/pants,TansyArron/pants,foursquare/pants,landism/pants,areitz/pants,benjyw/pants,sid-kap/pants,di0spyr0s/pants,mateor/pants,cevaris/pants,pgroudas/pants,wisechengyi/pants,dgomez10/xanon,fkorotkov/pants,jessrosenfield/pants,15Dkatz/pants,baroquebobcat/pants,twitter/pants,baroquebobcat/pants,baroquebobcat/pants,kwlzn/pants,cevaris/pants,manasapte/pants,megaserg/pants,foursquare/pants,pombredanne/pants,lahosken/pants,cevaris/pants,twitter/pants,15Dkatz/pants,benjyw/pants,lahosken/pants,sameerparekh/pants,gmalmquist/pants,Gabriel439/pants,tejal29/pants,sid-kap/pants,square/pants,foursquare/pants,15Dkatz/pants,manasapte/pants,ericzundel/pants,laurentgo/pants,dturner-tw/pants,slyphon/pants,qma/pants,di0spyr0s/pants,tdyas/pants,megaserg/pants,UnrememberMe/pants,twitter/pants,peiyuwang/pants,tdyas/pants,ericzundel/pants,Gabriel439/pants,sameerparekh/pants,ericzundel/pants,pgroudas/pants,TansyArron/pants,areitz/pants,TansyArron/pants,di0spyr0s/pants,digwanderlust/pants,ericzundel/pants,lahosken/pants,laurentgo/pants,jsirois/pants,qma/pants,slyphon/pants,jtrobec/pants,UnrememberMe/pants,di0spyr0s/pants,twitter/pants,pantsbuild/pants,dturner-tw/pants,Gabriel439/pants,dbentley/pants,kslundberg/pants,tdyas/pants,dbentley/pants,benjyw/pants,jsirois/pants,pgroudas/pants,megaserg/pants,twitter/pants,kwlzn/pants,pantsbuild/pants,jtrobec/pants,dgomez10/xanon,UnrememberMe/pants,pombredanne/pants,dbentley/pants,peiyuwang/pants,kslundberg/pants,fkorotkov/pants,jessrosenfield/pants,dturner-tw/pants,kwlzn/pants,digwanderlust/pants,mateor/pants,15Dkatz/pants,landism/pants,slyphon/pants,sameerparekh/pants,kslundberg/pants,foursquare/pants,pombredanne/pants,tejal29/pants,digwanderlust/pants,pombredanne/pants,UnrememberMe/pants,twitter/pants,areitz/pants,peiyuwang/pants,benjyw/pants,dgomez10/xanon,landism/pants,dturner-tw/pants,megaserg/pants,ity/pants,jtrobec/pants,pantsbuild/pants,twitter/pants,twitter/pants,pantsbuild/pants,15Dkatz/pants,kwlzn/pants,pgroudas/pants,lahosken/pants,jtrobec/pants,dgomez10/xanon,TansyArron/pants,jessrosenfield/pants,cevaris/pants,landism/pants,peiyuwang/pants,mateor/pants,scode/pants,dgomez10/xanon,jtrobec/pants,lahosken/pants,landism/pants,tdyas/pants,ericzundel/pants,peiyuwang/pants,sid-kap/pants,megaserg/pants,qma/pants,tdyas/pants,gmalmquist/pants,baroquebobcat/pants,scode/pants,dturner-tw/pants,lahosken/pants,jessrosenfield/pants,ity/pants,wisechengyi/pants,kwlzn/pants,ericzundel/pants,pantsbuild/pants,qma/pants,sid-kap/pants,cevaris/pants,scode/pants,dbentley/pants,landism/pants,Gabriel439/pants,laurentgo/pants,tejal29/pants,fkorotkov/pants,ericzundel/pants,laurentgo/pants,kslundberg/pants,foursquare/pants,ity/pants,benjyw/pants,megaserg/pants,areitz/pants,wisechengyi/pants,twitter/pants,scode/pants,mateor/pants,peiyuwang/pants,foursquare/pants,kslundberg/pants,manasapte/pants,manasapte/pants,slyphon/pants,kslundberg/pants,baroquebobcat/pants,kslundberg/pants,pombredanne/pants,qma/pants,scode/pants,scode/pants,pombredanne/pants,Gabriel439/pants,Gabriel439/pants,wisechengyi/pants,ity/pants,laurentgo/pants,kwlzn/pants,sid-kap/pants,jtrobec/pants,fkorotkov/pants,Gabriel439/pants,digwanderlust/pants,lahosken/pants,foursquare/pants,pantsbuild/pants,mateor/pants,lahosken/pants,gmalmquist/pants,baroquebobcat/pants,digwanderlust/pants,qma/pants,fkorotkov/pants,baroquebobcat/pants,UnrememberMe/pants,dturner-tw/pants,tdyas/pants,wisechengyi/pants,wisechengyi/pants,tejal29/pants,ity/pants,tdyas/pants,laurentgo/pants,ericzundel/pants,15Dkatz/pants,ity/pants,sid-kap/pants,manasapte/pants,UnrememberMe/pants,cevaris/pants,UnrememberMe/pants,manasapte/pants,peiyuwang/pants,areitz/pants,UnrememberMe/pants,pantsbuild/pants